There's other stuff like the Cruis'n trilogy, Animal Forest (for Japan users only), and the third party stuff like Bomberman, Mischief Makers, Ogre Battle 64, etc provided if Nintendo can get their respective owners to play ball.
The coolest thing would be for Nintendo to get the N64 and Arcade versions of the original trilogy to basically really allow a full experience of the series.
Though that might be my preference for getting the arcade versions of the main themes:
I, uh... guess everyone missed me about a page or so ago pointing out how Nintendo hasn't gotten around to adding the N64's OG Cruis'n trilogy onto NSO, huh?
Granted, I also mentioned how Cruis'n Velocity (for GBA) also isn't on GBA NSO. Although, does anyone expect THAT to be added (Velocity, I mean)?
Funny thing is, I honestly wouldn't have considered the possibility of Cruis'n coming to ANY NSO service, if it wasn't for Cruis'n Blast's Switch home port...
BTW, the reason Cruis'n Blast got a Switch port at all was because of the pandemic. Specifically, Eugene Jarvis (the series' creator) admitted that the arcade version's sales were REALLY being hurt by no one coming by to play the game (for understandable reasons), so the Switch port came about to recoup any potential losses.
Come to think of it, some Cruis'n content for Smash would be pretty cool. At least some music.
Oh, and maybe some content from Ridge Racer and Daytona USA (and other SEGA racing games) too.
I've talked about this quite a bit some time ago, but I'd actually LOVE a Cruis'n stage that serves as a sort of "remix" of the Big Blue and "Super Mute City" stages, where the cars are in the middle of a race while the characters are fighting on top of them.
Occasionally, the cars would perform tricks like flips, barrel rolls, "helicopters" and the like, launching characters off of them and forcing to find another car that's more "steady" to fight on.
Of course, I'd be perfectly fine with just Cruis'n music, as I imagine that Nintendo would need to write a pretty BIG paycheck to the various vehicle manufactures to have their vehicles featured in Smash Bros. (such as Chevrolet, Lamborghini, FIAT, etc.).
Speaking of music, I'd go nuts if Daytona's "Let's Go Away" theme song ever got into Smash. It'd almost certainly be an extra song on a Sonic stage, but I'd still take it all the same.
For that matter, Takenobu Mitsuyoshi doing Cruis'n music would also be a dream come true, as well...
Same goes for Ridge Racer music being on a PAC-MAN stage or Tekken (given R.R.'s connection to the latter series). Color me shocked if Reiko Nagase or Ai Fukami ever got Trophies/Spirits in Smash, though.