If you're considering picking up the new Pokémon games, which launch later this week, you may want to go back into Pokémon X & Y and make sure you've done everything you can in those games first.
Have you found all the mega stones? Done all the Looker Missions? There is a lot to keep track of in terms of post game in X & Y, but thankfully Reddit user technophonix1 made a handy checklist of all the things a completionist player might want to do in Pokémon X & Y. Take a look—you may want to click on "expand" to see this at full size.
In addition to this, it's worth remembering that the demo for Omega Ruby and Alpha Sapphire enables you to get a few extra goodies once you transfer over your information to the full game. So, you should play it if you're able to.