After defeating the Elite Four and becoming Champion, the game is far from over! You'll gain access to the Sevii Islands, where you can catch new Pokémon and complete additional quests. Completing the Sevii Islands story is crucial for unlocking the National Pokédex, which allows you to trade with Ruby/Sapphire/Emerald and catch more Pokémon. You can also re-challenge the Elite Four, who will have stronger teams. The battle against Mewtwo in Cerulean Cave also becomes available.
