As far as Pokemon goes, I doubt we'll see a newcomer in the base game, but if we do I believe it will be a Gen 8 Pokemon. They are distributing a "never before seen" Pokemon in Let's Go which comes out in November, which at this point would only make sense to be a "new" Pokemon, even if it is just a new form. So if we don't see a Gen 8 Pokemon on release, I think we will at least see one as DLC. Any Pokemon previous to Gen 8... I don't see it happening personally.
And I also don't think Captain Toad will be playable, as much as I would like him to be. Not because of Toad being part of Peach's moveset and not because he can't canonically jump, but just because he doesn't come across as a high priority character to me, especially given that his games are casual spinoff puzzle games.
Steve just straight up won't happen. But I wouldn't be mad if he did.
(my opinions)