On this topic though, I actually think a Metroidvania style adventure would be really fun in Smash Bros
Think, World of Light meets The Great Maze (which I know most people don't like either lol)
You start off with a super basic character, probably a newcomer dedicated to the role who can't use projectiles or any elemental attacks. As you progress, you unlock characters who can do those stuff, like unlike Mario, suddenly you can burn a plant blocking your path. Unlock Kirby, now you can cross a wide gap. Unlock Pikachu, now you can open doors that need power.
A premise like that I feel would be a fantastic platforming adventure while adhering to Sakurai's dislike of cutscenes.
Yes, Metroidvania Smash where characters are power-ups is something that could be really cool. There were a couple instances like that in SSE I think, I remember one treasure you could only reach with characters who could wall jump or had a super long recovery like Pit - but nothing more than that.
With this many characters I'm not sure it could work though, but once again World of Light was actually on the right track imo.
All the Spirits power-ups kinda ruin this since they will just nullify hazards, but doing some fights spiritless really asks you to think about each character's abilities in relation to a particular challenge.
The floor is lava? Pick Jigglypuff and just stay in the air!
There's an Assist Trophy who kills you in two hits? Pick Banjo and Kazooie and destroy it with a couple of Wonderwings.
Stuck in Shadow Moses, stamina mode, where you die in two hits? Pick Ridley, skewer the wall destroying it in one hit, then drag the opponent off-stage with side B.
Against four opponents in Magicant who all kill you in one hit? Pick Link, or any character with a big-ranged up air, and slowly chip at your opponents from the platform below.
Rathalos boss destroys your shield every time it hits it? Pick Marth and use his counter instead.
Etc.
The game turns into something like a puzzle fighter, it's really fun, even though some battles clearly weren't designed for this and just become incredibly hard and tedious.
But as a concept it's ace. You could easily use this in a WoL-like Adventure mode where you need to find the right character (or one of the right characters) before you can more strategically tackle a specific, particularly difficult challenge.
Also, steering too far away from traditional fighting and incorporating full-fledged platforming levels wouldn't really work with a cast as varied as Ultimate's in my opinion. Imagine having to do that with Steve.