Pokémon Mirror Gold: Beat Red on Mt. Silver
What opens up at this point in the game, and everything you can catch by now that you couldn't before.
Newly reachable
Mt. Silver 1FMt. Silver 3FMt. Silver 4FMt. Silver Expert Belt RoomMt. Silver Moltres RoomMt. Silver MountainsideMt. Silver Outside Pokémon CenterMt. Silver Top Snowy AreaRoute 28