Are we just to assume that as well as new ones all the old ones will be making a return...? As to new ones I'd like to see...
NINTENDO
----------
1) Wart (From Super Mario Bros. 2. Would be a big slow one like Bowser/Donkey.)
2) Kamek Wizard (From the Mario games. A floaty type character like Peach and such, but with more projectile like magic.)
3) Paper Mario (From the Paper Mario games. Uses paper abilities as specials.)
4) Olimar (From Pikmin. Summons Pikmins with different abilities for his attacks.)
5) Someone from Super Punch Out (A guy with pure physical supers, but brutish and powerful!)
THIRD PARTY
-------------
1) Slime (From the Dragon Quest games. Maybe not a normal slime, could be a knight, king, emeperor etc. If a normal slime it would slowly change colour through the fight, changing the elementals of its attacks. Like its projectile would change from ice to fire etc.)
2) Megaman (The original one. Guess there's 2 options here. Have him with his buster, slide, Rush attacks or have him a Kirby type. He does absorb enemy abilities, remember...)
3) Cloud (If you're going to have a Final Fantasy character it better be Cloud damn it! Just have him with the moves from his game. Link type, and his limit breaks easily fit the super moves.)
4) Yuna (Probably the second most widely known Final Fantasy charcter. Imagine each of her super moves unleashes some mini little chibi summon monster. It'd just be so cool!)
5) Goemon (From the Ganbare Goemon games. Coin throw, pipe smash etc. I love Goemon!)