Bring back wavedashing and l-canceling! (I wish ;-P)
In all seriousness, I'm not sure what they could add, but I would like them to keep things like Samus and ZSS being separate characters, and remove things like rage, which is a weird mechanic that shouldn't have been added.
"Just for comparison Uncharted 4 was 20x bigger than Splatoon 2. This shows the huge difference between Sony's first-party games and Nintendo's first-party games."