I've theorized about a mode like this for a New Smash game, or perhaps Ultimate DLC Expansion
Pretty much a mode where you kind of play a level from the represented franchises home games, complete with their Enemy mooks, Level design, and some of the bosses.
Pretty much a combination of something like Smash run, or some of the levels in Melee's Adventure mode, along with the addition of fighting bosses. it would not only be a very interesting representation of the franchises, but I think it would add alot to the Platforming aspect of Smash
Smash Ultimate as is really lacks a mode that focuses on the Platforming and movement aspects. Melee, Brawl, and Smash 3DS all had some type of mode that's like this. even 64 kind of had this with Board the Platforms. I also think Brawl and Smash 3DS were really interesting in particular because Subspace and Smash Run had you fighting Enemies that you usually don't fight in regular play, with their own Quirks and such, so it really separates itself more from the Multiplayer of Smash and is a very good breath of fresh air, as it's like a Beat-em up mode mixed with Platforming.
It's a big part of why I think the Single-Player in Ultimate is kind of weak in all aspects. the Types of modes in this game is not as varied as Melee, Brawl, and Smash 3ds