Mewtwo mission
After defeating Elite Four, he returns to Villa Pokémon. In the upper part of the area there is a cave that was previously inaccessible. Inside you will have the option to catch a Level 70 Mewtwo.
Legendary birds
After defeating Elite 4, one of the three legendary red and blue pokémon birds will appear in Kalos. According to your initial Pokémon, it will change: if you chose Froakie, you will see Moltres, if you chose Chespin you will see articuno, if you chose Fennekin you will see Zapdos.
After finding one of them, look at his position at the Kalos Pokedex coastal. You will see its current location. Walk – don’t fly – to the place challenges the bird again. After several attempts, the bird will retire to Spirit’s give where you can hunt it.
Safari
After defeating Elite 4, you can go to the Kiloude city and visit the Safari friend in the upper left corner of the map. For each friend code you have registered you can visit a different safari that gives the possibility of hunting a specific Pokémon. The variety increases based on the number of friends playing Pokémon X/Y.
This is a simple way to build your Pokédex.
Kiloude city
Defeat Elite 4 to unlock the Killouce city. After seeing the credits, you will receive a message from Professor Sycamore saying that he wants to see you at the Luminose station, where he will give you a train ticket for Kiloude.