The Looker quest and Mega Stones are basically the endgame as far as standard RPG "go do stuff" is concerned. Here's other "endgame" stuff you can do with all in game areas unlocked:
- Obtain all Kalos pokedex pokemon (save for the 2 legendary Birds not native to your region), then talk to the Director to receive 4 diplomas (one for each region, then)
- Breed berries (mostly useful for breeding pokemon as they are useful in reseting IVs)
- Breed pokemon and play other players online
- Maximized "style" in Lumoise for a few perks and to get the final O power
- Fight in the Battle Maison, alone or with friends. There are many items you can only buy with BP earned here (or, more slowly, at the Battle Institute).
- Obtain all pokemon, but with Pokebank out only in Japan this is still a tall order (even compared to normal). Receiving all non-event-exclusive pokemon should give another diploma from the Director like previous game