In this version of Emerald, which has been updated to offer only double battles (with the exception of trainers who only have one Pokémon), you can participate in the one-of-a-kind battles that are only available in this version of the game.
Every Pokémon, with the sole exception of the legendary Pokémon, is susceptible to capture in their natural habitat.
Don’t be shy about talking to the trainers before you try your luck at the Pokémon League; it has already demonstrated that it is more than deserving of its name at this point in the competition.