Pokémon Classic: Champion — the post-game
What opens up at this point in the game, and everything you can catch by now that you couldn't before.
✓ Cerulean Cave opens and Mewtwo waits on its bottom floor. Faraway Island, the Battle Tower turned into a shop for the Battle Points you win, Title Matches at the Indigo Plateau, a second team for every gym leader and then a post-game team on top of that, post-game Elite Four rosters, and Team Rocket fugitives to track down once a day. Beat the Champion a second time and you are given the Mega Ring; after that the Mega Stones start appearing around Kanto, with Mewtwonite X and Y on the second floor of Cerulean Cave.
