The awkward situation with Gen 9 right now is that up to this point, the pattern has more often been skip one generation and add the next. It happened with Gen 3, it happened with Gen 5. For the last two games, it didn't happen at all. At this point we could all agree that at least Gen 8 missed its chance, and now we're considering if Gen 9 may have missed theirs too. Worth noting this would end up glancing over the entire Switch era of Pokemon games, their big transitionary period over to home console and two of their best selling generations yet.
Gen 9 is placed at kind of the perfect time for consideration under the standards of any
other series, but this is Pokemon we're talking about and as
GoldenYuiitusin
pointed out they usually start looking ahead. But then I guess the question is, how far ahead can we look? When will Gen 10 come out, and when will Smash come out? There's a possibility that Smash and Gen 10 would overlap within roughly the same year, something a bit unprecedented as usually by the time they debut in Smash they've been around for a couple years. So I don't really know what to make of this yet, I think however we cut it the next game's Pokemon selection will have to be a little bit unorthodox.
For my current prediction I'll just lock in on a Gen 9 Pokemon tentatively then transition over when we know more about what's coming. I don't like leaving blank TBA slots even if that's what Sakurai himself does when plotting it out.