In respect to the indie discussion, and as cliche as it is to say, Undertale is so far beyond every other candidate that it's really not a contest. I'm fairly certain that if we get any character from an indie game, it will be Sans. I do specify Sans because he's easily the most popular choice and the one who they opted to make into a costume in the first place... one that inevitably sold the best out of any Mii Costume... but if you wanna slot in Frisk or Papyrus or whoever then go ahead.
Undertale's worldwide success is immense. It is so popular in Japan that, as cited in Toby's recent newsletter, they're simultaneously developing the Japanese translation of the next chapter of Deltarune presumably for a same day release. Toby Fox has reached somewhat of a minor celebrity status with a Famitsu column talking about localized games. I would go as far to say that out of any of the big western pulls right now (minus maybe like, Fortnite or Apex Legends?) Undertale has the most Japanese appeal. That's not even taking into account its standing against other indies... like, just in general. Shovel Knight and Shantae are left in the dust, I'm sorry to say. What they have currently is the extent of what I'm seeing for them moving forward.
I think Sans has a great chance of being playable. If he's not, I think Undertale will still see a significant increase in representation some way or another. It's one of the most notable and widely popular success stories of the last decade, in a game that probably serves to add more contemporary third party characters, and Toby Fox is in great standing with Nintendo and Sakurai already. And Undertale, in its eight years, has not really dipped in popularity whatsoever. I agree that indie games sometimes have a bit of a "shelf life", Shovel Knight support is not what it used to be, but Undertale is about as relevant now as it was when it released due to great branding, its viral presence and the rollout of its sequel.
It would be very difficult to argue such a persistent and definitive game in the online landscape between both western and Japanese audiences being a flash in the pan... I could definitely understand it during Ultimate, but now a whole four years later I think Undertale has made more than enough of a case for itself as an evergreen classic. That doesn't confirm Sans or an Undertale character in its own right, but I think it ought to be considered when weighing our options for "indie reps". One of the perks of an indie character is that negotiations are dealt with on a smaller scale, but most of the available characters are also on a smaller scale of notoriety or international appeal. That's why it's easy to get those ATs and spirits in, but I don't know if they perceived our current indie representation to be of a necessary scale. If Undertale is just as easy logistically and about 5x as popular, then that's a slam dunk I think. I take issue with the idea that it ought to remain at this supplemental level simply because it's classified as indie - by all other means, it's as popular as several third parties already playable.
Side note, this is neglecting the likes of Reimu when discussing Japanese popularity. Undertale and Touhou naturally have a lot in common, and even share the role of its music being a staple of rhythm arcade machines now. Both are wildly popular. I personally don't think Reimu has the necessary official western presence to be a strong contender but I figured it was worth pointing out I kinda put her in her own little box outside of the typical indie talk.