Post-game will always have less content than the actual game, that's a defined property of game design.
Like, look at Pokemon Mystery Dungeon Red/Blue and Time/Darkness. They all have what could be called a "Post-Game Story" because the credits role smack dab in the middle of the playable story, but people don't consider the middle credits to be the "End of the main game," far from it. I think that even if they did something similar to this in X and Y, people would still complain about Post-Game boringness, even if your Pokemon are at the max level.
I guess they did cater to the Post-Story players when they added the Battle Frontier in gen 3 and 4, but the problem with that was your Pokemon didn't ultimately grow as a result of the experience, making it seem like you didn't achieve anything. Same goes for the Pokemon World Tournament and the Battle Subway.
So here's some things I think they could do in order to make Post-Game a little better.
1: Rebattling Trainers
This has kind of been done before with the Vs. Seeker and (insert stereotypical Pokemon handheld device here) but I think they should expand on this a lotttt. If Pokemon is all about battling then you should have as many opportunities to battle as POSSIBLE, even in the post-game. Also, the levels of the trainer's Pokemon should progressively raise. Like, maybe everytime you beat a Trainer their Pokemon all gain two levels for the next time. That would certainly make grinding to level 100 a lot easier if you're in to that sort of thing.
2: Lots and lots of mini games
Mini Games are fun and they can make a game's life a lot longer. Yeah we have the Casino in some of the Pokemon games but that really isn't enough. I really liked the Pokemon Plaza back in Platinum, where you would go into a room with a bunch of people on Wifi in real time and you could interact with them and meet with them. That was genius and needs to be brought back. Speaking of which.
3: Real-Time player interaction over wifi.
Oh god Gamefreak was SOOOOOOO close. I thought the Entralink would literally be like, you could play your Pokemon adventure while tagging along with a friend. Unfortunately it didn't live up to my expectations but... Wouldn't it just be damn amazing if you could goof around the Kalos region with your friend and Double Battle random NPCs?
4: Secret Bases and Contests
3rd Gen was probably the best at keeping me entertained Post-Game and we need these back. Now.