I hope for a rework of certain older characters since ones like Samus could do with an update and for it to closer to Melee in terms of general mechanics. Ideally it'll have rollback netcode but I won't get my hopes up.
And I don't get this idea that they'll just re-release Ultimate on the successor since not only would that not sell anywhere near as much as a fully new game considering how much Ultimate has sold it would also not be a major system seller like a new Smash would be so it would be a bad business decision for Nintendo to use one of their biggest series like that for their next system.