It is a scenario with four ages, covering the Rise of Rome, Middle Ages, Renaissance, and what I called the "Imperial Age". (Beginning of industrial age, so there's coal and railroads but not really units from that era.)
It has 16 civilizations, 15 of them playable:
Rome (allied with England), locked in war with Carthage (and France), has units that are incredibly powerful and have terraforming capabilities, but their units are at their strongest soon after the beginning of the game. Though they retain relevance into the midgame, their Imperial-era units are not as good, and they lose their uniqueness down the line. Though all their units are strong, their Legionaries are their strongest.
Carthage (allied with France), locked in war with Rome (and England), has the cheapest unit line in the game and their War Elephants are their staple power, able to absolutely decimate opponents. They require gold/ivory/silver to build most of their units, but they are cheap, the idea being to overwhelm with sheer manpower advantage.
Greece, locked in war with Persia, has the best spearman-type units, as well as having early access to amphibious assault and hidden nationality units, making them adept at naval warfare. They also have access to certain otherwise-exclusive-to-Rome things in the midgame, owing to Byzantine heritage.
Persia (similar to Russia), locked in war with Greece, has a gigantic land mass, and similar diversity in units they can field, albeit exclusively early-focused compared to the late-focused Russia. They have Immortals who have 1 more attack than Hoplites but one less defense (so that whoever attacks has the advantage), but also have a wide array of units, including access to the Barbarian tree of units and being able to field Ansar Warriors. However, their units fall off in the mid to lategame.
All four of them, plus Egypt (who has the unique trait of chariots, but otherwise has nothing), have access to multiple techs at the beginning of the game.
England (allied with Rome), locked in war with France (and Carthage) is an Imperial lategame powerhouse, with the strongest naval units in the game, and second-strongest Imperial age units. Though they require getting to the lategame to get to their units, once they have access to them, they can absolutely dominate. They have both the strength (hard to reach), and weakness (hard to invade), of being on an island.
France (allied with Carthage), locked in war with England (and Rome) is an Imperial lategame powerhouse, with the strongest musket-aged units in the game. By sheer raw numbers, they have the strongest most powerful units in the game. Though they have a rough early game (given they need to survive the onslaught of Rome), if they can get through that, they can conquer the world.
Germany has nothing noteworthy to it until the very very very end, at the absolute last section of the tech tree. At which point they get access to three gamebreaking units, the Ironclad (better even than the English naval unit), the Artillery (best artillery unit in the game), and building 'mobile fortresses' (tanks), the best "Cavalry" in the game. However, they lack any special units until then, meaning that while they are the latest of lategame powerhouses, they have literally nothing until then.
Spain starts fairly weak at the beginning, but in the midgame gains access to one of the strongest units in the game; their Conquistador, with the full movement of an Explorer (buffed to be 4 movement with all terrain as roads), decent attack/defense, amphibious assault, blitz, and multiple other aspects, for the cheap cheap price of 30 shields (and they can be drafted, too), meaning that one unit can be built and spammed to basically sweep the map.
Portugal starts fairly weak, but has access from the getgo to a couple of minor Wonders that allow them to have sea capability. Their Carracks are some of the best naval transports in the game in being both warships and transports.
The Celts have the unique trait of all their units having one extra movement point. At their highest level, these units also ignore the movement cost of mountains, hills, and forests. They also have the most units capable of enslaving, giving them a dedicated workforce which can be sacrificed for cheap culture points. They also have access to the Barbarian line of units.
The Goths have the unique trait of sacrificing defense for pure sheer raw offense, having THE strongest overwhelming offensive advantage at every tier of their units, at the cost of having basically zero defense. (For instant, Teutonic Warriors at 10 shields have 4 attack, higher than any other unit's attack at that shield cost.) They also have access to the Barbarian line of units.
The Hordes have the unique trait of all of their units treating all terrain as if it were roads. Beyond that, they have some of the most diverse units, including horse archers, extra cavalry levels, incredibly powerful swordsmen, and all around have the strongest overwhelming potential with sheer mobility and strength. However, they start with the smallest amount of territory. They do have access to the Barbarian line of units to help them though.
The Vikings have access to some of the best midgame sea transports in the form of the Longship, while also being classified as a Barbarian civilization and having access to the ultimate unit from that line of units, the Beserk, an amphibious assault powerhouse with huge attack and decent defense and good mobility.
The Russians also have access to the Barbarian units and also have access to some Byzantine units while also having Imperial units of their own. However, while they have access to the Viking Beserk and Longship and Byzantine Dromons and Cataphracts, their Imperial units aren't the strongest. They have a huuuuuuge amount of landmass though, enough to rival Persia, so can remain relevant at every stage of the game.
The Turks have a somewhat sizeable amount of land, while having access to some incredibly powerful Imperial units. They have earlier access to Imperial age units, and while their Imperial units aren't as strong as France/England, they tend to have longer range, tend to have access to them earlier, allowing them to reach their peak power much sooner.
The idea is for each civilization to have unique strengths that allow them to dominate through fairly clear win conditions.
I'm working on putting all the units into the game.
Units put in have multiple levels. Initial level puts them into the game with all their final values and functions.
Then there's the next level which adds in their tech requirement.
The final form is renaming each unit to have a different PRTO and have a unique civilopedia entry, but this is a luxury.
I'm about 3/8ths of the way finished in this task.
Once I get all the units in at their initial levels (where they don't crash the game and are in with their final attributes), the next thing to do is add in all the buildings/Wonders that I want to add in. They, too, get an initial level and the next level which adds in their tech requirement, before a final form renaming each building to have a unique civilopedia entry.
Then comes doing the governments, tweaking them to be where I want them to be.
And then I tackle the tech tree. I have the techs, all of them, start to finish, worked out. I just need to put in the work there.
After the tech tree, I go back through the resources, all the units, all the buildings, and the governments, to set their tech requirement to the next level.
And then, I work on building the map for this scenario.
Once the map is done, the game is in a state where it's playable. It'd be messy, because it requires knowing all of the changes made due to the civilopedia not listing things correctly. But it'd be workable. (And the next step would be changes to making the civilopedia accurate. For the civilizations, for the units, for the wonders, all of it.)
It's a lot of work but I still am working on it gradually, bit by bit.