I'll give three right now, with the possibility of more later on:
1) Eevee. I love Eevee. She's one of my favorite pokemon, and part of it is because of the Eeveelutions but I also like Eevee on her own. My idea for an Eevee fighter is to have probably the Down special attack be Evolve, where Eevee would evolve into one of the Eeveelutions (It would either be based off of the stage- each stage would have its own "element" attached to it; Able to cycle through a la Shulk's Monado, so you'd choose a stone to have your eevee evolve into; or you'd cycle through all of them like the Pokemon trainer). Each Eeveelution would work like a semi-clone of Eevee, so only the special moves would change between eeveelutions with all of the regular moves being the same.
2) Gengar. Gengar has always been one of my favorite Pokemon, and the Ghost type is by far my most favorite type. Also, with his inclusion and character portrayed in Pokken tournament I'm even more in love with this Pokemon. Gengar has all kinds of potential, is very important to Pokemon history, and is still a well designed Pokemon.
3) Here's where I get whacky: Exeggcute. Exeggcute will never be playable, but having the Egg Pokemon make an appearance as a fighter would be great. This is partly because I love Exeggcute and it's line (An Exeggcutor was among the Pokemon who helped me defeat the Elite Four back in Red/Blue. It was a good Pokemon). But with it's grass/psychic pairing there are a lot of great things you can do with the character. Oh, and before you yell "How would Exeggcute use items?", they'd either do the Mewtwo thing where they use their psychic power to hold items, or one of the egg/seeds (probably the one who's head is burst open and is real sad looking) would just hold the item in their mouth and use the item (like, with the Home Run Bat for instance, the other egg/seeds would just swing this egg at opponents).