Well, we got both Pokemon Trainer and Lucario in Brawl. I feel like the situation is a bit different now, but maybe there's a possibility of adding a Pokemon from the previous generation on top of the most current one at the time of release?
I guess it boils down to whatever is inspiring Sakurai - truth be told, if they're onto Gen 10 and they want a Pokemon from there then I don't know why Sakurai would fall back on someone like Meowscarada unless he was really captivated by the idea. Pokemon Trainer was a novel concept so it was probably being driven more by that than anything else. So it'd have to be a pretty significant, inspiring case... As far as Gen 9 goes I've been digging Ogerpon from what I know about them, maybe he'd be into the mask thing.
Or something from Gen 5 if there's a remake, I dunno. FireRed was relevant when Brawl was being decided. Maybe it's finally Zoroark's time to shine...