My guess is that if there is a Nintendo character, it's going to be a pick similar to Byleth. With this in mind, it's likely something that happened in this Nintendo Direct, which means no Kirby. Instead, we're looking at:
- Famicom Detective Club: The Missing Heir
- Famicom Detective Club: The Girl Who Stands Behind
- Mario Golf: Super Rush
- Miitopia
- Animal Crossing: New Horizons
- Hyrule Warriors: Age of Calamity
- The Legend of Zelda: Skyward Sword
- Splatoon 3
As a revival, the Famicom Detective Club games sound like a decent bet, but there's also not really any sort of indication that they specifically want it to flourish, so I wouldn't put many chips in that corner. Mario Golf: Super Rush pretty much just has Waluigi as a candidate, and also it's Mario Golf; Probably not getting anything there. Miitopia was low key before and it's a miracle that it's getting a port now. The Mii Fighters are already a thing anyhow so I doubt it'll get a character even if it was the next big thing.
Animal Crossing: New Horizons is probably the biggest thing here, but it's also already had it's promotional pick, so probably not. The Legend of Zelda series also already has a whole bunch of stuff, and it's not likely to get any more candidates to join unless they want Zelda. Splatoon 3 is coming out next year, so it's probably not getting anything here either.
There's a
tiny possibility that they might monkey's paw in Mathew to celebrate a Golden Sun revival if they indeed intend to revive the series, but other than that, I don't think there are very many other likely candidates unless they haven't shown off their big game that isn't The Legend of Zelda: Breath of the Wild's sequel yet for whatever reason.