There really is no telling for what will and won't be included. What they included in SwSh was kind of random. The DLC of course brought over a lot of good competitive Pokemon which was great. For the main game I think it will be similar to SwSh. A mix of popular and just random Pokemon. A popular Pokemon is not necessarily a competitive one.I wouldnt count either the "popular" and "good competitive" pokemon as reasoning for a inclusion of a Pokemon in the game.
Notably for example, lord and savior Greninja was missing from SwSh, and several competitive behemots were missing well over until the 2nd DLC came in.
Honestly I think they just go with whatever they think fits the area.
One thing is for sure, they will of course include Pikachu because that is essentially the mascot of the franchise. And I assume they will include the Gen 1 starting Pokemon as well, too iconic for the franchise. I do hope they bring over all starting Pokemon but I doubt they will. Probably will reverse the ones from SwSh. Gen 1, Gen 2, Gen 4, Gen 5, Gen 6 and Gen 9. But if they're bringing over Hisui Decidueye maybe all Gen 7 starters will return again.