You could always try post game things like going to the top floor of Black Tower/White Tree Hollow. (Can't remember other post game stuff haha)
Alternatively you could try and complete your Pokedex and trade a lot, you could start to play competitively and raise crazy strong teams, or you can restart your game and take on a challenge.
However in the end, it's all up to you. That's just some suggestions I can throw your way.