You had Jump Ultimate Stars which had chars from over 40 different manga but that game sadly never came out of Japan and the newer jump stars games went for a weird 3D battle style. JUS was like a combination of a normal 2D fighter and Smash, you had actual life and actual combos but also things like ringouts and items and platforming and what have you. Also you had incredible amounts of crossovers. You could have Kenshin and Luffy and Goku and Yugi all in the same team since you could pick from up to 4 different chars and switch them out on the spot mid-fight.
But yeah for more recent times I guess Dissidia counts? It's all FF but most of em take place in different universes (though apparently they're going hard on VII and X being in the same world according to the remake easter egg lol).
But see, my point is you either have a game which is about those various crossovers like Smash or the vs Capcom stuff, or you just have bad games using these guests as crutches to rely on, which is my estimation of what KI and MK are doing. I won't lie, seeing battletoads in a fighting game looks hype, but then you see how it plays and as a competitive player it just turns me off instantly lol. The ACTUALLY good fighters don't need to rely on those things, they're just in-universe chars like GG and BB and Persona arena and so on.
