The way I see it, every Smash game has an element that inspired its development.
Smash 64 - Bring together some of Nintendo’s heroes in a fighting game
Smash Melee - Build on what Smash 64 was, and show more of Nintendo’s history
Smash Brawl -Try to make a memorable, epic experience that people will get excited about just as much as they would play it
Smash 4 - Try to bring a little bit of what people liked about each Smash game, and put it all in one
Smash Ultimate - Make the ultimate Smash Bros game, which will by extension become the greatest videogame crossover ever.
If I made a Smash Ultimate sequel, it would prioritize finding new experiences no Smash game has before while keeping what people like about the series. I’ve noticed not a lot of games have really made that their goal. This isn’t to say Smash games haven’t been trying to make new experiences, I’m just saying that’s probably the direction the series should go in