Pokemon could be really interesting as an open world game, and it would be very doable. Each area could have Pokemon of a certain range of levels, so some areas would be more difficult than others. It'd be up to the player to either challenge themselves and go in with an underleveled team, or to stick to areas they can handle and make gradual progress. The gyms could be done in any order, but every time you beat one, the other gyms become more difficult just by level scaling. The story would have to be re-worked so you discover Team Rocket's grand schemes piece-by-piece, or something like that. The story is simple enough, it shouldn't be hard. And I think becoming an open world game would help Pokemon feel more like your own, personal adventure.