So, potentially new topic- or at least one I haven't really seen.
I know some have suggested a Ring Fit Trainer for a character- but I honestly think that players of the game, as well as people who would potentially be exposed and buy it, would find that dull (also, the Ring Con can be a very annoying character).
However, there is a character that I think would hype Ring Fit Adventure, as well as be fun in Smash- I'm referring to the bro dragon himself (Brogan?), Dragaux.
The game, RFA, has outsold even Animal Crossing in some regions during certain months, and it's become a game heavily associated with the Switch platform, particularly with the Blue Ocean (untapped) market.
Dragaux himself, even for those who don't like the trainer, would be a marketable character- a dragon with muscles who taunts you.
While I don't think this is a likely character, I do think this would be the way to go if RFA were to get a character (not to mention, another Switch era game would be given a nod).