Some baseline RPG rules are in effect.
You have enough item storage to store every item in the game (though, there is a quick-select menu of nine items), and the limit for carrying is 99.
You have a party of nine in this case, which can be subdivided into parties of three as needed for specific areas.
And at the beginning of the game, you get to select which class each of your characters are. (Optional would be selecting gender.) A generic group of heroes, so characters don't have unique roles to fill--character A doesn't need to do something only character A can do; there's no real backstory or much in the way of plot interaction. The heroes go places and do heroic stuff, in a world which in spite of this lack of characterization is still rich, filled to the brim with an internal lore.
Or something to that effect, where the world itself is important, and the actual features of the game are important (it being a game), but beyond that, nothing is important because those two aspects take front and center stage.
To start out with, the game has eight stats:
Attack, Defense, Magic Attack, Magic Defense, Speed, HP, MP, and Ki.
Attack increases damage of physical attacks dealt; Defense reduces damage of physical attacks received; Magic Attack increases damage of magical attacks dealt; Magic Defense reduces damage of magical attacks received; HP is how much health the character has; MP is how much mana the character has; Speed controls how rapidly the character's action bar regenerates and chance to evade attacks. All standard. The standout is Ki, which both increases damage of Ki attacks dealt and decreases damage of Ki attacks received.
Each stat has a normal cap of 99, multiplied by 10 for MP (999) and 100 for HP (9999). Pretty standard.
Every class has at least one "starred" stat. Starred stats not only grow faster than other stats (mind you, certain classes gain stats at different rates even if not starred but a starred stat is guaranteed to grow faster than any other non-starred stat), but grow further, up to double that of the normal cap: 199, multiplied by 10 for MP as 1999, multiplied by 100 for HP as 19999.
Equippable items can raise stats above their normal cap, up to the starred cap level, albeit rather difficult to get. (Most equippable items aren't, saaaaaay, +50 to a stat; +5 would be more akin to the range we're talking.)
However, battle consumable items, and/or spells, can raise stats up for that battle to be above the starred cap level--to the absolute maximum of 999, multiplied by 10 for MP as 9999, multiplied by 100 for HP as 99999. But, as these effects wear off at the end of the battle, and would be requiring an insanely high amount of time to reach, doing this is mostly for the sake of doing it; there's no pragmatic reason to boost stats beyond the starred maximum, or for that matter, to a large extent, past the normal maximum.
To keep it simple, you can differentiate things as being "quest items" (have their own item menu), "unique items" (cannot be sold, cannot be thrown, cannot be duplicated, etc.), "consumable items", and then equips, done into three categories, of Weapon (no class is required to hold a weapon; some classes can't use some weapons; all classes have preferred weapons), Armor (no class is required to wear armor; classes can't use armor heavier than their stated type but can use lighter, but are best in their preferred armor), and Accessories (not sure if it'd be just one equippable accessory or 2-3 slots for this, but if multiple, it absolutely would be utterly interchangeable; every class except martial artists can equip any accessory).
There's also an "Offhand" slot, for holding a second Weapon which provides half the effect of something in the main hand. Not all weapons can be in the offhand, and some weapons are two-handed, eating up the offhand slot.
Then there are 25 playable classes, subdivided into 9 categories, from which the number of players is derived.
The three "White Mage" classes (Holy), the three "Red Mage" classes (Balanced), the three "Black Mage" classes (Demon), the three "Fighter" classes (well, technically, three "Warrior" classes, but Fighter is the most well-known name even though it's not the center; these are the light melee), the three "Knight" classes (heavy melee), the three "Archer" classes (ranged), the three "Rogue" classes (throwing), the three "Thief" classes (sneaky), and the Martial Artist.
Each class has a reason to be used, unique to them.
White Mages are, along with Black Mages, unique among the 'class namers' (unless you also count the Fighter), in being at the far end of the spectrum in their class tree. White Mages are the ultimate healers, having access to every healing spell in the game. They also have access to every buff spell in the game, and have some minor debuff spells in their arsenal as well.
They are not, however, exclusively medics, though, because they also have access to powerful Holy magic: light-based, lightning-based, sun-based, etc. Spells which are illuminating, fiery, bright, and the like. Their preferred weapon is staves; they cannot use large blades (small blades, i.e., daggers, are fine). They can wear only 'Robes', the armor class which is so low it's almost nonexistent.
Their starred stats--yes, plural--are MP and Magic Defense, making them the ultimate answer to defending against magical assaults. They are rather squishy, however, vulnerable to being attacked.
Clerics are the next step in the White Mage tree--they have access to most of the healing spells in the game, and all the Holy offensive magic that White Mages have, including most buffs (but not all). They can wear the Light Armor class, allowing them to have a greater array of defense. They also have a much, much stronger, notable, melee attack, and in fact, can actually attack and have it do notable damage. Their preferred weapons are large blades; they can use staves or small blades as well.
They do not have a starred stat as of right now. They also have no debuff spells.
Paladins are even further--they only have a few healing spells, and only about half the Holy offensive magic in the game, with zero buff or debuff abilities. In exchange for this, they can wear Heavy Armor, and are very strong melee fighters, to the point where their melee is stronger than their magic. They are also built like tanks, capable of taking a huge punishment. They have the starred stat of Defense.
Red Mages are pretty much what you'd expect: they can use about half the healing spells in the game, fewer than a Cleric but a lot more than the Paladin. They can use some (de)buffs, but not the full array. They also have access to every Holy offensive magic in the game. On the flip side, they can use every Black Mage spell, except the debuff and status ailments ones.
They have a few other advantages as well--they have the second-highest stat growth in every stat, and can use any weapon (including none)--and are the second-best at using said weapon of any class. (While not as good as, say, a Fighter with a sword, they are second only to said Fighter in the sword; while not as good as an Archer with a Bow, they are second only to the Archer for the bow; while not as good as a Martial Artist barehanded, they are second only to the Martial Artist barehanded, etc.)
There's also their unique quirk, which the other two RM classes lack: they, and they alone, have the ability to cast spells at a reduced cost; say a fireball for a black mage costs 10 MP; it'd cost the red mage only 8 MP to cast. (75%, rounded up.) They can wear light armor; their starred stat is MP.
Blue Mages, to the left of the tree on Red Mages, have the same base effects of a Red Mage: same spells (albeit lacking debuffs or buffs), high stat growth, proficiency in the same weapons, etc. Though lacking the reduced mana casting ability, they make up for it with their own unique quirk: the ability to learn enemy spells. This makes having one a necessity, since certain spells in the game can only be used by enemies, and a blue mage is thus required for the purposes of learning them all. They also share the starred stat of MP as a red mage.
Crimson Mages, to the right on the tree of Red Mages, are basically a Red Mage on steroids: they have the same spells as a Red/Blue Mage, with one notable exception: a complete lack of so much as a single healing spell. They more than make up for it, with their unique blood magic. Blood Magic spells use both HP and MP to cast, but are unique spells that have unique effects, including at least one status ailment unique to the class that not even the Black Mage (master of status ailments) possesses.
Among said spells are also ways of generating MP at the cost of HP. One spell, acting as a barrier, amplifies damage received by 3x, but causes 1/3rd of all damage received to be absorbed as HP. The Crimson Mage is also one of the very few classes to possess a Mana Drain spell, directly siphoning enemy MP into the Crimson Mage. They also share the same class traits as the Red Mage:
Proficiency with every weapon (in fact, even exceeding the Red Mage in that regard), high stat growth, ability to wear Heavy Armor (albeit preferring Light), and similar amplifications. There is a downside, though. In spite of this increased attack power both physical (hitting almost as hard as the melee classes and harder than the paladin) and magical, spells cost 3-5 times their normal MP. Add in a lack of starred stat plus the complete absence of healing magic, and the crimson mage while basically unequaled in ability to dish out damage (exceeding even Black Mage abilities), has no way to prevent damage from being dealt.
Black Mages are pretty much exactly what you'd expect: every offensive magic that's not Holy, they can cast. Every debuff spell not classified as Holy, they can cast. Every status ailment in the game, they can cast. Including being one of only two classes with the Instant Death ability! Their other properties mirror that of the white mage; they can only wear robes, use staves, and have the starred stat of MP. However, instead of a second starred stat being magical defense, the Black Mage's second starred stat is Magical Attack.
Battle Mages, the next on the BM line, are also pretty much what you'd expect: wearing Light Armor, and able to use long blades (but preferring small blades), they can use every black magic spell...except for the debuffs and status ailments. They have significantly higher ability to attack and take a hit back, but pay for it in having no starred stats.
Spellblades, the last on the BM line, may not be what you'd expect: able to wear heavy armor, they're more like an equal mixture of death knight, vampire, and spellblade. Their normal, baseline attack will drain MP if their target's Defense is higher than their target's Magic Defense--the strongest, easiest to use MP drain technique in the game.
Since every spell they cast goes through their blade, this produces a unique effect: if their target's Defense stat is lower than their Magic Defense, then the attack will be classified as a physical attack; if their target's Magic Defense stat is lower than their Defense stat, then the attack will be classified as a magical attack; if their target's Defense and Magic Defense stats are identical, then the power of the spellblade attack used is doubled.
They also are unique among all the classes in being the only class to feature a Life Leech spell, draining HP from their target and siphoning it back into them. They can only use longer blades, and obviously their spells use up MP and are weaker than their non-bladed cousins (a fire sword isn't gonna have the same impact as a fireball), but this still makes them able to answer to any situation. They lack a starred stat, however.
Warriors, the center of the Fighter column, represent a balance of Light Armor melee fighters: equally focused on defense and offense, they are pretty run-of-the-mill. They have starred stats of Attack and Defense.
Barbarians, the left side of the Fighter class, represent abandonment of armor and defense altogether in favor of pure offense. They can't wear armor, but have the highest attack stat growth with Attack and Speed as a starred stat, and a class-based immunity to all status ailments and debuffs except Berserk.
Fighters, the right side of their titular class, represent a focus more on the defensive, with starred stats of both Attack and Defense, but a much higher growth in HP and Def than warriors have. They also possess the immunity to debuffs, albeit vulnerable to status ailments.
Knights, the center of their class, are Heavy Armor, heavy-hitting individuals, with the starred stats of Defense, HP, and Attack.
Samurai, the left of the Knight classes, are also Heavy Armor, but focus more on Speed, with the starred stats of HP, Speed, and Attack.
Dragoons, the right of the Knight classes, share this Heavy Armor, but focus on attack, being one of the heaviest hitting melee classes and also one of the heaviest armored, albeit at the extreme penalty of speed. Slow-moving, but with starred stats of HP, Defense, and Attack, and having gains there higher than any other classes do.
Archers, center of their class, are ranged-based fighters, preferring the use of bows. They can wear light armor, and are generally able to fight from afar, chaining multiple attacks with speed. Their starred stat is precisely that, Speed, and of the Archer classes, that is their highest gain.
Rangers, left of that, are basically archers with less speed and attack, but with the aided benefit of being able to use magic--a few white mage spells, and a couple black mage spells, including a status ailment spell or two. This comes at the cost of having no starred stat.
Gunmen, right of archers, are basically archers with a heavy emphasis on chaining together powerful attacks in rapid succession with their guns (their preferred weapon). They have the starred stats of Attack and Speed, but pay for this in being unable to wear armor.
Rogues, center of their class of "throwers", have the ability to throw items--thrown items do increased damage (or in the case of armors, damage at all) and may have effects or increased effects in the case of thrown consumables. This is an ability all three share; the Rogues unique spin on this? Every item thrown is treated as if it were sold, giving the party gold equal to that item's sell value. They have the starred stat of speed, and use small blades and wear light armor.
Ninjas, to the left of that, copy pretty much the same stats as Rogues albeit not hitting as hard (lower attack growth) and lower defense. However, this is because their ability more than makes up for it; they have the power of doubling Items (specifically Items, not Consumables). That is, when Throwing an item, their Throw can be doubled in strength; their Throw can be thrown at two different targets rather than just the one; their Throw can not consume the originally thrown item effectively giving them unlimited supply of that item. They have the starred stat of speed as well, also wielding the same equipment.
Alchemists use the same equipment and have the same starred stat with similar stat growths to the ninja, and have the inverse ability: they have the power of doubling Consumables. That is, when Throwing a consumable, their Throw can double the effect of that consumable, double the number of targets for the consumable, or not consume the originally thrown consumable effectively turning, say, one HP potion into an infinite supply of them. (However, this latter ability does have the coded-in inability to duplicate an MP-generating consumable. All these abilities? Yeah, they use MP, and this is one loophole not available to to the players. I thought of that.)
Yes, these are precisely as strong as you'd imagine them to be.
Then there's the remaining classes.
Thief is another must-have in a 100% completion game, for being the only class with the Steal ability, able of stealing Items, Consumables, and Gold. Since some opponents have unique items/consumables only possible to obtain via this method, a party wanting every item needs a Thief. They have similar capacities to a Rogue, in just about every regard otherwise.
Duelist, to the left, is basically a melee rogue instead of a throwing rogue: light armor, but focused on Speed and Attack (both starred) to deliver deadly blows. Light on HP, but having innate evade that's the highest in the game, they're ridiculously hard to hit, and also rather lethal.
Assassins, the final of that branch, are masters of status ailments almost rivaling black mages, to the point of being the other holder of the instant death status ailment. They focus on pure Speed and Attack (also both starred), but sacrifice their innate evade while being the lowest of the three on HP.
And finally, the Martial Artist class.
The Martial Artist class is unique in not being able to equip anything: no weapons, no armor, no accessories. They make up for it in having stat growth almost on par with red mages in all stats, and being the only class with a starred stat growth in Ki--in fact, the only class at all that even can have their Ki stat naturally increase.
Martial Artists specialize in incredibly powerful blows, chained together in rapid succession, dishing out multiple hits per turn that all pack the largest punch in the game at baseline levels. There is a drawback, however--whereas other classes gain weapons that have innate stats that are higher (thus, meaning that their attacks with a new sword do more damage than their attacks with the old sword in spite of having their own personal stats be the same), martial artists being unarmed fighters have no such luck.
They make up for this with being able to use Ki attacks. Ki attack use mana, but have the unique property of being classified as neither physical nor magical in nature--allowing them to be one of only two classes (the other being the spellblader) to bypass opponents who have high stats in both Defense and Magical Defense. It gets better. Most opponents have next-to-zero resistance against Ki attacks. The few who do are usually quite vulnerable to physical attacks.
Plus, the few enemies making use of Ki attacks would devastate your party normally but a Martial Artist with that as their starred stat is unfazed by this. The tradeoff is, as mentioned, the lack of equips--in any given RPG, you know why that is hugely detrimental, as by the lategame, you have epic items that are capable of the, like, +50 stat gain, or items granting a multitude of permanent buffs, or items granting immunity to debuffs, or status ailments, and so on and so forth...and they can't use any of those.
You can probably tell that some of these I haven't gotten to think about properly and that the system isn't quite perfected, but I think that it's not bad for less than an hour's work in dreaming up, and would in fact be perfectly at home as a Final Fantasyesque game.