Here is a list of players I think will be in it:
1. Mario
2. Luigi
3. Peach
4. Bowser
5. Bowser Jr./Shadow Mario
6. Yoshi
7. Donkey Kong
8. Diddy Kong
9. King K. Rool
10. Wario
11. Kat & Ana
12. Link
13. Zelda/Sheik
14. Ganondorf
15. Toon Link
16. Toon Zelda villain (perhaps Vaati if he ever appears as a villain in another Zelda game. But by then, there's bound to be at least one new Toon Zelda game, and I believe the villain from that game should be playable)
17. Pikachu
18. Jigglypuff
19. Pokemon Trainer
20. Lucario
21. Psychic Pokemon
22. Kirby
23. Meta Knight
24. King Dedede
25. Gooey (from Kirby 3, Gooey could use his copying abilities for his side special move, and could copy characters' side special moves)
26. Fox
27. Falco
28. Krystal
29. Wolf
30. Samus/Zero Suit Samus
31. Ridley (unless there's a new recurring Metroid villain)
32. Marth
33. Ike
34. New Fire Emblem lord (preferably a spell caster, but NOT another swordsman)
35. Ness
36. Lucas
37. Masked Man (Claus)
38. Captain Falcon
39. Samurai Goroh
40. Pit
41. Ice Climbers
42. Mr. Game & Watch
43. R.O.B.
44. Sukapon (unless there's a better choice for a retro WTF-character)
45. Captain Olimar
46. Will (from newest Advance Wars; will most likely become a recurring character in the series)
47. Isaac
48. Ray
49. Snake
50. Sonic
51. Mega Man
52. Bomberman