My idea: After you beat the maingame there is a huge island sealed off from ordinary everyday trainers that contains many powerful Pokémon, hugr expansive dungeons and harsh varying climates. Your rival decides to go there to train and isn't heard from for over a week. You gather a team of strong trainers from all around the region to rescue him/explore. This island would have many powerful items(mega stones, etc.) and would serve as a location for alot of legendaries to be caught postgame.