I don't think it's entirely out of the question, as mid-gen introductions are a thing they've been doing since Gen III, with entirely new Pokemon from a dex standpoint from Gen VI, so we might see some new Legendary that Arceus created around then or something. I expect it'd be more than one though, and tbh I'm expecting a different form for Arceus more than I am something entirely new.
i personally think there won't be any new pokemon and obviously a very cut pokedex. It says we are gonna make the first Sinnoh pokedex, therefore I think we will mainly encounter pokemon we have already met in the Sinnoh region
Probably not. It's Game Freak, making a kind of game they've not made before, expectations for certain things need to be kept low.
From a lore standpoint it wouldn't be weird to see "new" Pokemon or new forms though. While we don't know exactly how long ago L:A takes place, it's likely long enough ago that it would be possible to see Pokemon or forms of certain Pokemon that were only around at that time in Sinnoh and went extinct or changed into a new form or moved elsewhere by the time of modern Sinnoh in DPPt.
“No, I... I have to be strong. Everyone expects me to."
There's more than likely going to be no new Pokémon.
After all, it is Sinnoh just at an earlier point in time.
It'd be unusual to make new Pokémon that are then not around during D/P
While I do agree that we probably won't see any new pokemon, I think they could easily make it work to have a new pokemon native to Sinnoh.
With the release of BDSP they've got a free pass to rewrite/add in new elements to sinnoh, so they could easily have a new ancient event pokemon in Legends and then have a small event in BDSP where the focus is that a super old pokemon has been discovered