There's been a lot of prediction-making so far, I see no harm in adding fuel to the fire.
SPOILER ALERT: This is a wall of text!
What we do know is that Cloud will be joining the roster with two other fighters, a Final Fantasy stage will be added along with two more - having never played FF at all I won't know where it is, and at least one new Mii Fighter costume; the Chocobo Hat. Using the past year as a blueprint and an predicting algorithm I've giving it very serious thought about what to expect, but before I do I must point out that this is mostly speculation and should not be taken as any kind of fact.
As I've mentioned a few times before, even with Cloud's reveal I think at least one veteran will return. It's a bit of a question of simple deduction, I've always felt and still fell that Wolf is the most likely to return; in part there is an argument to be made for Wolf, but a lot of my argument is based largely on the remaining veteran cast, as I'll now demonstrate;
- 50%: Wolf and Snake are the most likely to return in my opinion. Having said that Wolf seems to be the more likely of the two. Yes, Wolf is a semi-clone, a reinterpretation of Fox. However his unique standard moveset, agility and dominating air game balanced with his poor recovery and almost fatal meteor cancelling gives Wolf a distinct difference between him and Fox and at a bit of a disadvantage with Falco, at least at face value.
- 45%: Snake is the other very likely veteran to return. Snake's unique fight style is a major plus point and in and ideal world Snake would join Wolf - in fact that can still well happen, however unlikely. Most of his attacks are high damage, high knockback and a surprisingly camper playstyle.
- 3%: Just about every other veteran is almost completely discounted, but a few of them do have plus points worth noting. Of the remaining Pokémon Ivysaur is the most likely to return, the argument can be made that Ivysaur has potential by virtue of his type; no other Pokémon available is a Grass or Poison type. However Ivysaur as a fighter is an average attacker and a poor defender with a horrible recovery.
- 1%: The Ice Climbers would be much more likely if it weren't for the fact that Sakurai more or less discounted their appearance on 3DS. Personally I think the Ice Climbers have their unique double-hit mechanic and fightstyle is unique enough to warrant a return.
- 1%: In theory Squirtle is a better fighter than Ivysaur, but there's already a fast, light, Water type Pokémon in the form for Greninja. Despite that Squirtle's moveset is unique and should not be hastily passed.
- 0%: Toon Link adequately replaces the role Young Link provided in Melee as an alternative Link fighter, a third Link character would be a very bad idea, especially when Young Link has disadvantages Toon Link doesn't possess.
- 0%: Sakurai made it clear that he didn't want interchangeable fighters this time around, the separation of Zelda/Shiek and Samus/ZZS is proof of that. There's no reason Pokémon Trainer costumes can't appear for Mii fighters and I hope it happens, but with separate fighters and the fact that Charizard is his own fighter the Trainer won't return.
- 0%: I could argue that Pichu suffers terrible disadvantages against Pikachu, nevermind the Melee roster. I could argue that Pichu had no advantageous matchups against any other fighter in Melee. However the argument I'm taking is simply that noone wants Pichu; there's only one pro Pichu fighter right now and only five before him. Rock on Mew2King!
As for newcomers it's pot luck! There's no real point in speculating. I would like Rayman to appear but I've never been bothered with whoever's revealed, and you are talking to the one person who voted in the Fighter Ballot for Commander Keen. However there does also appear to be a pattern for newcomers, specifically which newcomers won't be revealed.
If you've been following the Mii Fighter costumes you'll know that Mii Fighters can dress up as other characters. Some of these character costumes are unique and others are from current fighters in the roster. There is a pattern with the latter fighter costumes, they are all costumes of veteran fighters - Link, Samus, Fox and Captain Falcon - every other character costume is that of a character not on the roster. While it's not a concrete theory it's reasonable to assume that Mega Man X, Proto Man, Dunban, Isabelle, MegaMan.EXE, Zero, the Inklings, Akira, Jacky, Heihachi, K. K. Slider, King K. Rool, Chrom, the Black Knight, Llyod, Toad or Viridi won't be revealed as newcomers based on this conclusion. This is just a theory though, based on observation, and should not be considered as fact.