I'm still sticking with Reimu. Smash has slowly but surely been adding more indie content, and with how accepting Nintendo's been to indies lately, I'd be surprised if we don't come out without one. Sure, we couldn't get in Shantae, Shovel Knight, or Sans ****ing Undertale, but those are Western characters who came to prominence recently. The Touhou Project, on the other hand, is a long-running Japanese series, so I'm thinking/hoping that Reimu will succeed where others fell short based on having different merits than them.
It helps that Touhou has a fanbase like no other, to the point that some people find the series through fanworks and don't even realize they're a series of bullet hell games. It's gotten to the point that the standard for Touhou remixes is 60 to
200 per song, and it dominated conventions to the point that it had to get it's own all to itself in the form of Reitaisai, which was held annually until this year because, y'know, COVID. Heck, the only reason the games are left untranslated on Steam is because ZUN knows that the fans will make their own anyways.
Also Suika somehow managed to crash ESPN once. That actually happened once. Just sayin'.
I'm also betting on Isaac as the Banjo of FP2. He's popular, he's got a series to back him up, but hasn't been seen in a good long while. It helps that the backlash to his snowglobing made Nintendo aware of his popularity, if they weren't before.
Lastly, I've been taken aback by Sakura Wars' popularity in Japan. It's been adapted into just about every form of media, gets showered with praise from fans and critics, dominated a 2016 for revival interest and general brand interest (Competition included Jet Set Radio for the former and
Sonic the Hedgehog himself for the latter), and even got it's own themed cafe and
25 stage shows. Plus, it's recently been branching out to the West, with games such as Project X Zone and the 2019 soft reboot. If Nintendo's okay with focusing on Japan specifically-Or not, if they or Sega decide they want to push Sakura Wars in the West like with Dragon Quest-I think Sakura has a pretty good shot.