Question

What can I do after beating the Elite 4 in Emerald?

Answer 1

After you defeat the Elite 4, you can battle the champion.

After you defeat the champion and get the national Pokedex, there are a bunch of things you can do.
Explore Altering Cave, Desert Underpass, Trainer Hill, SS Tidal, Artisan Cave, and the new Safari Zone areas
Complete all the Trick House puzzles
Receive more berries on Route 123
Obtain a Meowth and a Sudowoodo from the Battle Frontier
Catch Kyogre, Groudon, and either Latias or Latios
Rematch every trainer in the Pokenav (including gym leaders) and Wally
Defeat Steven in Meteor Falls
Get all the gold symbols from the Battle Frontier
Win every master rank contest and get every painting in the Lilycove museum
Complete the Hoenn Pokedex (if you can trade)
Complete the national Pokedex (if you can trade even more)
Obtain as many different items as possible
Obtain as many different ribbons as possible
Obtain as many different secret base items as possible
Pick up all the items on the ground
Visit Mirage Island
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Answer 2

There is the Battle Frontier...

Answer 3

Like coolunown said, there's the Battle Frontier to battle in, and you can also EV train Pokemon to fight competetively and fight with other people near you that still play Emerald / Sapphire / Ruby / Fire Red / Leaf Green.
Or you could always save your game for stuff and import if you ever get DS 4th generation games.

Comments

Like meeeeeee(I have emerald & fire red/leaf green & a lost ruby & sapphire

Answer 4

If you complete your Hoenn Pokedex, you can get one of the Johto Starters from Professer Birch.

Comments

* The entire Hoenn Pokedex excluding Jirachi and Deoxys.

Answer 5

how about trying to catch Rayquaza, Groundon, and the three legendary Regis. :)