Sort of on-topic wrt predictions. No clue when it'll come (def not this early in the Switch 2's life) but I am entirely convinced that the Switch 2's smash title is just gonna be Smash Ultimate Deluxe. Since the Switch 2 has backwards compatibility, a hypothetical Smash 6 would have to compete with SSBU. I don't see Nintendo wanting to put in the effort to try to compete with the biggest smash game, so I'm fully expecting a full priced Switch 2 version of Ultimate with maybe a few new characters, maps, and a half-assed game mode.
You know what, I'd be fine with this. I think I can go a console generation without a real Smash. The series was dragged out for damn near a decade since Wii U, I think it can be put to rest for a little bit - might even improve Directs if they don't all devolve into Smash sperging for 2/3rds the console's life.
As mentioned previously, I think Sakurai needs to walk away. The series has been his prison since 2012, I think he's due to retire it and work on projects he actually cares about.
Best thing for a Switch 2 edition would be to just fill out the roster with some missing links, make it easy and just leave out 3rd parties from the newcomers roster. Add Bandana Dee and Dixie Kong to round out their casts. For Mario, you got either Pauline or Toad/Toadette still to build out the roster. Add some new Zelda character that isn't the trio, bets on Impa, Skull Kid or a Wilds era character. Then round it off with a surprise character like Rythm Heaven or something. Getting like 5-10 characters is good enough, just make them all from different Nintendo series to diversify a bit.
From there, a few new stages to go alongside the characters. Maybe add in locations people have been clamoring for that never made it, like Bowser's Castle. Personally, would love to see a stage based on the Edward Island Manor from Another Code as a Luigi's Mansion sort of stage. An Endless Ocean stage would also be nice - I just really want more bizarre IPs like we got with Electoplankton back in Brawl.
In terms on new modes, don't even know what you could do. Would like another Subspace, but that seems too big for this. Maybe a Melee style Adventure? Trophies coming back would be a great, low-effort, edition if nothing else. Something felt lost without them.