Oh I’m sure he’ll show up in Paldea to team up with the new kids to deal with whatever the mystery Legendary in the crater is. Unless he’s rotating (which is possible) It’ll be Pikachu plus five Paldea guys. Maybe while in the crater he’ll capture one of the Paradox Pokémon to make it six. (I just like every region having a full team to represent).True to that is if he does go to Paldea if not well i am not going to say anything alright?
If Ash does i'll be happy for one more series but eh? possibly not.
Frankly via Ash now being sent off to Off-Screen Land I could easily see his Kanto collection reach two dozen (not counting the rest of the bull herd here). The power of nostalgia is strong with Pokémon. Though factoring in cross-region evolutions they could double as members of other teams if they evolve. Plus regional variants like Hisui Arcanine or Alola Sandslash.
As for the other regions? barring legendary/mythical/UB’s/etc. I could see every region from Johto-Galar with 4 more captures at most.