I don't see Meowth being in the background as an indicator of old pokémon being in the region. The meowth appears to belong to the protagonist and his family, and they just moved there. If I look at the Japanese trailer where the family moves in from Japan and pair that with the appearance of Meowth, I'm actually fairly certain they moved there from Kanto... would be a nice little throwback as well for the 20th anniversary.
So yeah; just because they brought their beloved pet pokémon, doesn't mean they also imported a bunch of wild pokemon. I think there'll be an emphasis on new pokémon; again, it's the 20th anniversary, I feel that deserves a nice amount of new mons!