Galgavias
Koopa
There's definitely an argument that leaving the series alone for a bit will be good from the prospective of freshening up the roster. In six to eight years there'll be new IPs to choose from, new favourites in established series, and new third party partnerships struck. I've said before it might be good for Smash to 'Third strike' its roster and refocus on a smaller group of predominantly newer icons to keep things fresh. Waiting for a new generation of characters that a future Smash can pull from may be a smart move.
In the end I think Nintendo won't want to wait long for the next Smash, and for all this talk, I don't think fans will want to wait long either. You can look at Mario Kart 8 for a similar situation in which its been seven and a half years since its original release with no sequel in sight. Despite a Deluxe edition and mobile game launching in the intervening time, fans are now beginning to seriously itch for a new one. Even if a complete edition of Ultimate launches on the next Nintendo console, I give it five years before fans start to ask where the next one is.
In the end I think Nintendo won't want to wait long for the next Smash, and for all this talk, I don't think fans will want to wait long either. You can look at Mario Kart 8 for a similar situation in which its been seven and a half years since its original release with no sequel in sight. Despite a Deluxe edition and mobile game launching in the intervening time, fans are now beginning to seriously itch for a new one. Even if a complete edition of Ultimate launches on the next Nintendo console, I give it five years before fans start to ask where the next one is.