Hmm, it's hard to say what the full list could be, but among contenders for unique; Isabelle, Isaac, Geno, Steve?(the issue is that Microsoft getting a character isn't that hard, but the best sales for them would be DLC. Steve? is the only one who is already pretty much heavily on Nintendo where it isn't a necessity to be pure business), Banjo & Kazooie(though now I see why people think they'll be DLC. I'm not nearly that confident in them), Master Chief(same reasoning among Microsoft characters as earlier, though base game would still make sense since it's an active franchise, not just purely a fan favorite, which would sell far better as DLC, especially if it's a dead franchise), a new Pokemon(Gen III or VII seems the most likely, though who it'll be is hard to say. Gardevoir might be the only option left if we consider a Pokemon already shown off or among the stages are disconfirmed. Gen VII has 5 potential ones, being Lycanroc(any form), Incineroar, Decidueye, Buzzwole, and Tapu Koko), and that's kind of it. Zeraora missed the deadline hard as is. DLC or nada, and a Gen VII base roster is way more likely anyway. Gen VIII has a reasonable possibility as DLC down the line, though, and that's all I can think of that are fairly likely. Rayman and/or Rabbids are a contender, but possibly a smaller one. I don't think Shantae will make it to playable, but I think she'll show up as an AT or at least a Trophy. I actually forgot Skull Kid and Elma. Skull Kid I'm not that confident on, as it feels like a lot of strawgrasping for him to be in(at least based upon the chairs), but he definitely is higher than before with the AT missing and other various tweets, plus a good amount of popularity. Probably ballot too. Elma I think has a good chance, due to Xenoblade being rather popular and the timing works. Never played the series, so it's a character I sleep on unintentionally.
For Echoes, there's Shadow and Ken for fairly likely ones. Lesser but potential options are Felix(for Isaac), Impa(for Shek), Dixie Kong(for Diddy), Ninten(for Ness). Dixie is also the only one among these who has a decent chance for unique. As much as I want to see Impa unique, I doubt it'll happen. HW is a spin-off, so is likely off the table for moveset potential(spin-offs tend to not get referenced like that. However, they certainly can give inspiration to show a character can potentially fight). Oh, and Octolings. How'd I forget them? Derp.
DLC is another matter. Rex & Pyra are possible(though it also depends if they find an interesting way to make them work well and if Elma is already base, though her being base wouldn't cut into them, but it's hard to say if he wants to give them that many characters just yet. Sakurai has made it clear that the amount of series representation, not just playable characters either, but in general, is taken into consideration). Spring Man is the only one I feel fairly confident in.