In positive news, I actually think the Deluxe Miis we got this go around (Sans, Cuphead, Vault Boy, and Bomberman) are all priority picks for the next Smash.
I know people have been refuting this, and I do think it's a bit premature to say they're outright
priority picks, but I do think that Bomberman and Sans have very good shots of getting into Smash at some point in the eventual future.
Bomberman feels like a matter of time, as long as Konami stays on board and Smash continues down the path of implementing third parties. If we have to scale back on third parties at some point then it all gets pretty shaky on who ends up being a priority in general, although I think Bomberman is a character whose history justifies him either way.
I'm going to defend the claim that Sans is already a viable choice for the future also. I think Undertale being an indie game clouds some people's perspective of how massive the game has become - it's one of the only western indie titles to achieve an equal outpour of love from both the west and Japan (even scoring a special concert for its 5th anniversary in Japan), the soundtrack continues to hit unreal numbers of viewership (a
reupload of Megalovania from about two years ago has already hit 60M, I swear there was one that was nearing 200M that isn't around anymore)... and I mean, the reaction to the Sans costume speaks for itself, which contested that of full characters and eclipses every other costume by a mile. Say what you will about whether or not that excitement was genuine or driven by the memes, but no doubt Sans and Undertale are still insanely popular (as stated by Sakurai himself, Sans was supposedly a pretty popular request).
And Deltarune is coming within the next few years, and is pretty much a guaranteed success from such a dedicated audience, so Undertale does have a fairly definitive future. Even if we weren't assuming that Undertale has already cemented a huge legacy for itself among some of the most impactful games of the last decade, and Sans establishing himself as a legitimate gaming icon. Sure, Undertale doesn't have a lot of games, but I wouldn't call it small by any stretch of the imagination. It's scale comes in other forms and it's been pretty monumental.
Cuphead and Vault Boy... well, Cuphead is arguably a bit smaller than Undertale, so I feel like it's an uphill climb trying to beat out Sans (or Shovel Knight for that matter). We'll have to see what the future holds for Cuphead, I'd personally like to see him make it someday. And then Vault Boy doesn't really strike me as someone who ought to be playable in the first place. He makes a great mascot Mii Costume, but nobody is begging for him to get promoted.