This looks fun!
First off, I think we will get plenty of DLC, as we know a DLC shop was found in the 3DS ROM data. I don't see the purpose of that if we are only getting 2-3 characters.
55% S Tier: I don't feel confident in claiming anythings above a 50% chance when it comes to Sakurai anymore
40% A Tier: Wolf and Lucas: Obvious picks are obvious, the fact that their custom moves are in the game shows that at least some work went into them before they were scrapped, and they would be fairly easy to bring back.
25% B Tier: Cap Toad, Dixie Kong, Isaac, Bandana Dee, Inkling: Toad, Dixie Kong, and Dee are all important to their respective franchises, as well as being pretty popular. Inkling could advertise Splatoon, and all Golden Sun content being erased feels suspicious to me.
15% C Tier: Snake, Rayman, Sceptile, Pichu, K. Rool, something from Zelda, another Mii Fighter: Kojima still wants Snake in the game, and that's really the only reason he got into Brawl. Rayman's already in the game, and I can't see why Ubisoft would settle for just a trophy for their mascot character. Sceptile's a relevant Pokemon and a grass starter, while Pichu would be stupidly easy to make (Maybe bundled together?) K Rool is popular but not as important as of now, I think Zelda will probably get somebody I just don't know who (Impa, Ganon, Toon Zelda, new character, I dunno), and more customization options for the Mii's (Mages coming to mind) seems like something that would sell well to the general public.
5% D Tier: Ridley, Squirtle, Ivysaur, Roy: The only reason Ridley is here is, as of now, he's still the only Metroid character that makes sense, no other Metroid character compares when it comes to star power. The fact that he's been mentioned by Sakurai and other Nintendo representatives shows that he's on their mind, and Sakurai has changed his mind before. If the next Metroid game protrays Ridley in an easier to translate to playable role, I'd bump him up to B Tier. Squirtle and Ivysaur are Pokemon, which gives them automatic star power, but I can't shake the feeling that they were killed off alongside the Pokemon Trainer (Who I forgot to put on here, he'd be on C Tier as a reworked standalone character) when Charizard was split off. Roy's had his role replaced by Lucina, but he still has a fanbase.
1% E Tier: Prof Layton, Krystal, Chorus Men, Ice Climbers, Chibi Robo, Wonder Red, another Metroid character: Layton seems to be the next likely 3rd party newcomer after Rayman, although that isn't saying much, I doubt Krystal would get in over Wolf and I don't see us getting 2 Star Fox DLC, Chorus Men and Ice Climbers solely depend on finding a solution to the 3DS limitations, Chibi Robo and Wonder Red aren't big enough names that I feel safe putting them any higher, and there is little to no chance of them adding in any other Metroid character over Ridley until said character appears in another game. (Imagine them putting in Count Bleck over Bowser, or Drawcia over Dedede)
.00001% F Tier: Mewtwo and Banjo Kazooie: Banjo Kazooie's chances are tragically impossible. Pffffft, Mewtwo, he's never coming back!