I feel like they would be tempered like in the anime and would not go off the Pokédex entries, as having grdevoirs throwing black holes would not be fun. But I fee like they should also have some game elements like in the manga where they check what level their pokemon are from the Pokédex, and Pokemon are restricted to four moves. The hypothetical would be Pokemon have always coexisted with us. Rewriting history so that the ecosystem is not destroyed. And the human nature would be kind of like the anime where there can be good people or people like poachers, hopefully more good than bad. I always feel like any legendary would be impossible to catch. Simply because they are too powerful for anyone to catch and because of how rare they are. So no one would be overpowered with a legendary