One idea I had involved a Pokemon-style game where you capture and collect monsters...but unlike Pokemon, you don't have limited moves on them, you can have allllllllll their skills, both passive ones and active ones, as they level up. Furthermore, furthering this, you wouldn't be fighting 1v1 or even 2v2. You'd be fielding a full, RPG-style, team of five, in all fights, against enemies that can range in number from 1 to 10 (with trainer v trainer battles being 5v5). Yet you'd still have the backup mechanic. Many RPGs allow you to substitute in party members who're out of combat, into combat. (Say you have 5 party members but mechanics limit you to 3, you can sub in any of the 2 out at any time, is what I mean.) This would be no different; you could sub in any of your equipped arsenal of trained monsters, up to 20 at a time.
Furthering this: while the monsters don't have head-to-toe human equipment (hand, offhand, helmet, armor, leggings, boots, etc.), they're not limited to one item; they can equip up to three.
But even furthering it would be two special bond mechanics: monsters you field not only bond with you, but bond with each other as they fight as a team. Bonding with each other unlocks unique combo moves only useable by those with that bond, and bonds with the trainer allow for similar things in the pokemon games but also are what allows for the monsters to use limit breaks. (This game idea was inspired a bit by Persona games, but even Persona games have a limit on Persona's movesets and limit the bond mechanics; this game would fully exploit them.)
Another idea I had was an RPG game where equipment would level up, separate to the characters: Helmets, Armor, Leggings, Weapons, and Accessories would all, universally, start at Level 1, and gain new stats as they leveled up and in some cases, gaining new status effects. (Notably, the game would feature some equipment that can be reforged and/or merged, with the reforge/merge being, universally, nothing but an upgrade with no downside, no content missed from it, nothing locked out from doing it, only a good thing, to preserve the levels of older equipment but give it refurbishments to upgrade it and allow for its expanded use.)
But sadly the idea, while a neat one, requires a lot more depth than what I can give it.