Here's my dream roster, without getting too crazy and adding a completely ridiculous amount of characters:
Before E3, I wouldn't have added either Villager or Wii Fit Trainer(much less even thought of her), but now that they're in, I see no point in leaving them out - I suppose they've grown on me. Glad I got some official art for Megaman, though!
Fire Emblem has always been one of the trickier series for me to pick characters from, but I think My Unit, who I should really rename either Robin or Tactician, would satisfy a lot of peoples' desires - wouldn't be yet another blue-haired swordsman, could use magic, and could possibly have both male and female versions.
Pokemon is also a little tricky, given how many there are too choose from, but I think bringing Mewtwo back with his new form incorporated would work well.
Zelda gets 7 characters because... I couldn't decide between Skull Kid and Vaati, so screw it, they're both in
I'm sure a lot of people will think I'm crazy for not adding Shulk, but, eh... I wouldn't mind seeing him get in, but I honestly think people way overestimate his chances, and he's just not among my top picks.
Also, no Toad or Bowser Jr. because I've just never been convinced that they would be that interesting.
If I wasn't trying to restrict myself with 3rd parties, I'd totally add Quote from Cave Story and Gilius Thunderhead from Golden Axe, because
robots and dwarves!
60 characters is still pretty unrealistic, imo, but that's what wishlists are for, I guess.