I have a theory that we might not get a Gen 9 for another year or two, and that Legends might actually be "Gen 9". I believe instead that we are closer to a Gen 5 sequel or remake.

1. The Diamond and Pearl clans (while BKP and other Poke Tubers seem to think otherwise) have a distinct divide between characters that reference Gen 4 in the Diamond Clan, while Pearl Clan represents the newer generations.
   We see this in that the Diamond Clan has Arezu/Mars, Mai/Marley, and Sabi/Cheryl. Iscan and Melli are a little harder to pin down, but I think that at the very least Melli might be Lucian based on hair texture and colour, while Iscan is just completely lost on me. Might just be completely unrelated at this point.
   Meanwhile, Pearl Clan literally has Ingo, Lian/Clay, and Palina/Burgh. While many point to Gaeric being Wulfric, I believe it's equally likely to be Brycen since he has more Japanese influences. As for Calaba, I just don't see the Mina connections that Poketubers are making. Eyes being tired-looking don't make a solid enough connection.
2. We see Alder-looking images in both the Diamond and Pearl tents.
3. The only non-Sinnoh Pokemon to show up are Unovan.
4. Gen 8 also heavily referenced Unova throughout the game, and also features Garbador's Gigantamax which to my knowledge is the only non-Galarian or Kantonian Gigantamax.

None of this makes a solid guarantee, but I find an interesting pattern nonetheless.