Melee Adventure Mode was boring and repetitive. SSE had horrible level design which made it mostly unfun to play the first time, so playing it over and over again was uninteresting, but at least you only had to play through it fully once, then play a few stages a second time, to unlock everything.
An SSE with better level design, gameplay that sticks more closely to the multiplayer, a more Metroidvania style, and more challenge altogether would be acceptable. It doesn't really have to be shorter, as long as the actual stages are individually more fun to play, having more of them is actually a good thing.
Personally, I want stickers to return, and I want Sticker Match to be a multiplayer option. The ability to customize your character's stats could make for a whole new tournament environment. Having to balance larger boosts against a greater variety of boosts, as well as the metagame of what your opponent is likely to choose would add a lot.
An SSE with better level design, gameplay that sticks more closely to the multiplayer, a more Metroidvania style, and more challenge altogether would be acceptable. It doesn't really have to be shorter, as long as the actual stages are individually more fun to play, having more of them is actually a good thing.
Personally, I want stickers to return, and I want Sticker Match to be a multiplayer option. The ability to customize your character's stats could make for a whole new tournament environment. Having to balance larger boosts against a greater variety of boosts, as well as the metagame of what your opponent is likely to choose would add a lot.