Alright, so, this may seem like a really obvious question to ask, but I wanted to just put this forward in a much more direct manner and see what ppl say.
Newcomers you think have a good shot for next game. Go.
For now this is what I see as very likely:
First party
-Meowscarada: Others have already mentioned it, you need a new Pokémon rep, it's just that big. And as of right now, Meowscarada is imo the frontrunner, it got huge moveset potential, is very popular, it will be the first gen 9 mon of Unite, and it will scratch the itch of those who wanted a solo grass starter.
-Octoling: I'm not sure if they're going to do Octoling as an echo or a semi clone, but I'm sure they'll throw it in there, it just seems way too obvious with how big Splatoon is and it probably wouldn't take that much development time.
-Tom Nook: Like Spen18 said, this one is often slept on, but with how big New Horizons I actually think he's a front runner.
-Mio: Xenoblade 3 was pretty big and we know Sakurai loves jrpg stuff, I'm sure if he's back he's going to heavily considered her. If Sakurai isn't back her chances might be a bit lower, but I still think she have a decent shot either way.
-Sidon: The community is dying for more Zelda character, and Sidon is really popular right now. Would make a lot of sense considering the succes of Totk.
-Waluigi and/or King Boo: Yes I'm unironically predicting Waluigi. Sure nothing as really change for his cases, but he's still a huge request so maybe they'll do it this time? Also King boo as another Mario rep because Luigi mansion 3 was absolutely massive (16th best selling Switch game).
-Bandana Dee: Not gonna lie, I was never confident in his chances back in the Ultimate era, but with the big succes of Forgotten land and his roles becoming bigger in each game, I think his chances are now way better.
Third party:
-Crash Bandicoot: Huge request, and I actually think that him being soon (most likely) a Microsoft characters will help his cases a lot
-Jill Valentine and/or Leon Kennedy and/or Chris Redfield and/or Claire Redfield: Yeah not sure how they'll pull it off (Echo fighters, Skins like Hero), but I'm confident we could have at least one or maybe multiple RE characters. The franchise is absolutely massive with Village and 4 remake, and Capcom seems very easy to deal with. Honorable mentions to Dante and the Monster hunter, but I think RE is the most likely.
-Something from Sega: Could be a second Sonic rep or Arle, but I could also see Ichiban Kasuga or Alis Landale as a dark hose pick.