Alrighty-then, here goes nothing. I know there might be a lot on here, and it's in no particular order (except the bottom row being "Secret Characters").
If you need any clarification on why I didn't put in X or I did put in Y or how come Z isn't a Secret Character, go right ahead and ask.
Additionally, I figured Mega Evos would be utilized as transformations or finishers or supers, so I did not include them in here. However, I suppose they could be secret characters.
Anyway, I'm going to first explain why my "Secret Characters" are separate from the rest. I chose them basically on the fact that they are either unique to the point of gimmicky, or legendary. Or both. Here they are:
>Darmanitan: Zen Mode would give him some unique options and allow him to change playstyle mid-match, going from a brawler to a distance fighter.
>Eevee: He plays differently from other fighters because he is quadrupedal, first and foremost. Additionally, all of his Specials would be based on Eeveelutions, as he would evolve and then revert back, much like Red's Vee from Adventures.
>Meloetta: You can switch Forme mid-match with Relic Song, and her playstyle will switch between a close-quarters brawler to more of a spacer-type character.
>Deoxys: Formes. Formes everywhere. But seriously, the ability to utilize multiple Formes in battle would be very interesting, and would give him a nice mix-up game.
>Aegislash: Obviously would play rather uniquely when just factoring in his body shape. Stance Change and King's Shield would make for an interesting fighting game character.
>Mewtwo: Because Mewtwo and 2 Megas.
>Staraptor: An aerial fighter is something rather unique and interesting, and Staraptor does fit the bill for a fighting game with moves like Close Combat, Brave Bird, and Double-Edge.
>Keldeo: Quadrupedal fighter once again makes his playstyle rather unique, and he's a legendary, so Secret Character.