For first party promotional picks, it's honestly hard to say right now. Quite a bit of people on here have been talking about a new Zelda rep who could promote BOTW2, but if you're talking about a character from Zelda in general, then there's plenty of choices. But if you're talking about a character from BOTW2, well we can only make guesses based on what we know, because all we've saw from it is the teaser at the end of Nintendo's E3 Direct. That game's Zelda could be a candidate, but we don't know what role she'll have, whether she'll be a major supporting character as usual or end up being a playable character in BOTW2. Or you know we could have the game's version of Ganondorf. We'll have to wait until more details from BOTW2 are shown in a presentation before we have a better picture on who could represent that game.
As for Pokemon, it's a confusion situation to be honest. Yes, we got SwSh spirits back in November, but we only got four spirits from that, and two of those spirits consisted of two groups of Pokemon. What we could see are fully evolved forms of one of the starters or another new Pokemon like Toxtricity. Or we could see a Pokemon from SwSh's DLC as the new Pokemon rep. This will still be debated for months to come.
But if we're thinking a character who could promote a new game like what Byleth was intended to do for Three Houses (he/she was chosen before his game's release, which later became the highest selling game in the Fire Emblem franchise before he/she got in), then maybe we could guess someone like Sylux due to him being rumored to appear in Metroid Prime 4. Oddly enough, he's not present as a spirit even though a couple other bounty hunters from Hunters (Kanden and Trace). Let's not take this as a sign, of course, since anything can happen and we don't know about Metroid Prime 4's release window other than the fact that it had to restart development and Retro is still hiring in certain positions.
And of course there are sleeper picks like a character from Ring Fit Adventure. But in the end, Nintendo chose the characters in the second Fighters Pass, so it could be anyone. We are speculating on newcomers, after all.