While I love the idea of Smash Ultimate bringing all the characters back, for Smash 6 I would take a smaller roster in exchange for a new concept and more diverse playstyles. I would like to see an art style that mixes Ultimate and the darker tones of Brawl. To me, Ultimate looks crisper and cleaner (probably due to console graphic capacities improving over the years) but Brawl had a dark edginess suitable for fighting games. As far as the engine and mechanics, I'm not sure. It seems like Ultimate is aiming at ease for less skilled players while having a technical capacity acceptable for more skilled players. I think that's ideal. They seem to have updated the technical aspects of the game quite a bit, but I haven't played it at all yet, so I can't say if I'd want even more edits for 6.
As far as characters, I'd be willing to sacrifice ones I like to get more diversity. For instance, I think characters like Dr. Mario, Daisy, Young Link, Toon Link, Lucina, Roy, Dark Pit, Pichu, and Wolf (and maybe even Falco and Lucas) can be cut. While they have (or will have) traits to differentiate them from their similar counterparts, they are just too similar. I'd rather see more underrated Nintendo IPs and third party reps with completely unique movesets. Along the same lines, I think automatic veterans (i.e. Mario, Link, Donkey Kong, Samus, etc.) could be given substantial updates to make them also somewhat new while still being familiar.