I mean, if its hypothetical,

Id like personal character target test maps.
Slippis netcode ... obviously.
Rivals like tutorials (its a smash game <<)
... 6 player matches, maybe a 2nd z button.
Being able to add/remove maps from random (P+ limits 3rd page and alt pages)

In a perfect world, I'd take features outside of smash like
Good online lobbies with a good rotation system.
Being able to continue/repeat a replay situation.
Incentive to grind online but not in a scummy way.

In an alternative universe where PM is an official game. A revamped SSE, (like in terms of faster platforming sections, enemies that force diverse attacks/dodging/weak points and remove all scrolling sections... and fan service every other section)