A follow up to smash ultimate will be very difficult for several reasons.
I can't imagine them being able to pull off bringing as many characters in a sequel and that by itself might make it look like an inferior version no matter other potential upgrades.
I don't think Sakurai will return to the series anytime soon and he is for many the face of smash.
There are very few things to improve in ultimate, the online experience is the only part of the game that feels lacking. And I do not see that changing anytime soon since Nintendo are stubborn as hell when it comes to allow certain things (like a working voice chat or more focus on competition) and still have a lot to learn when it comes to building an online experience in general.
I think the series might wait a generation until the next new installment. Even a port or something like an HD-upgrade of Ultimate might be difficult to release due to the licensing nightmare I can imagine.
However, the game is such a money maker that it will be difficult keeping it off the market for long.