Oh what the hey, I'll throw my two cents into the fray.
- Extremely doubtful. Sakurai's confirmed that Toon Link is his spiritual successor, and it would seem a bit much to have another Link in the roster. I don't doubt he could have a unique and varied playstyle (using the power of the masks, various other items from the series, etc.), but I still think I'd rather just see a decloned Toon Link. I'd see him as an alt of Toon Link before anything else. His purpose has already been fulfilled in another character's playstyle.
- Definitely the most likely. I'm not going to expect to see him anytime soon or anything, but of the three Melee vets, he's definitely got the highest chance, especially if he is decloned. Extremely low chance of appearing as an alt. Definitely going to be his own character if he ever returns.
- Veeeeryyyyy veeeeryyyy doubtful. His role is essentially that of a last-minute joke character and isn't really necessary to the game for that reason. He definitely has a cool mechanic, though, and that essential premise could definitely be used for another character. Maybe he could be an alt of Pikachu, if his proportions and hitboxes are reassessed. Has a unique niche in terms of his mechanic, but there are better options to wield it, and Pichu doesn't really have much purpose anymore.
EDIT: Just to clarify, if they were to appear in a manner akin to Doc or Lucina - i.e. last-minute clone characters to pad out the roster / appeal to their fans - then I could see them happening and would be pretty cool for it to happen.