Speaking of stage, what kind of new stage gimmicks do you want developers to add?
Soft-scrolling stages.
What I mean by this is stages that scroll at the pace of the players, instead of automatic scrolling. Picture how Subspace Emissary handles Co-op, where the camera tries to keep both players in focus while still moving forward as progress is made through the level.
In this scenario, I'd imagine
all players would have to be on the right half of the stage for it to scroll further right. The game should account for everyone, not just one person, so the guy playing Sonic can't NSMB-Speedrun everyone and constantly move the arena faster than anyone else can keep pace.
I think this would be a good solution to make scrolling stages more intuitive/palatable. The screen moves forward only when it's convenient for everyone. Maybe the stage could even have actual automatic scrolling when the players aren't moving it, but it's so slow that it just subtly moves the arena forward without causing too much trouble.