Personally, I've never agreed with the whole mindset of "Smash Ultimate is the most amazing thing ever and can never be topped!" or that "Cuts will make the next Smash game automatically worse!"
I mean, while the roster is obviously a huge part of Smash, there are still other factors that matter, and when it comes to non-roster stuff, Smash Ultimate has plenty of room for improvement. The most obvious example of this is Ultimate's doodoo-tier online. A Smash 6 that improves the online with a better quickplay and/or ranking system, no preferred rules BS, more options and modes, etc would be greatly appreciated. Another great thig to see would be a single player mode with actual story (unlike WoL), and have actually good gameplay (unlike SSE). Seeing Smash Run and some variation or new take on break the targets would be nice. Give me a classic mode where items AREN'T involved, and where there aren't matches where I have to face off against godtier AI team while I'm given dumb worthless AI teammate.
Also, I don't think topping even UItimate's roster would necessarily be impossible. I won't pretend that I liked every character inclusion or thought every single one was good, or that there still aren't plenty of notable characters yet to be added. For me, how much I like a hypothetical Smash 6 roster would come down to multiple factors, like how many characters get cut, who gets cut, what franchises, if any, are lost on the playable roster side of thing, what happens to characters that are cut, what types of characters do we lose (ie antagonists, RPG character, platformer character, etc), what newcomers do we get to replace what is cut, what gameplay modes get added to make up for cuts, do assist actually have a bigger better role, and probably a crapload more factors or questions.
Depending on the answers to the above, I could very well see a different roster with some cuts maybe still being pretty nice. And even if the roster of characters ends up being not as cool as Ultimate's, if the gameplay aspect is much better in alot of areas then I'll likely still buy and greatly enjoy the game.