What if this is the game we asked for years that you could explore all the existant regions? It seems to start with only Kanto and the first 151 Pokémon, but GO also started with 151 and started expanding the amount until having 386 Pokémon so far.

Maybe Johto, Hoenn, Sinnoh, Unova, Unova 2, Kalos, and Alola will be added later through DLC.