There's plenty to do afterwards.
Excluding Gen II, it's remakes, FireRed and LeafGreen and Black and White 2 which have a bunch of cities to visit afterwards, there may not be much to do afterward, but same with all the other games.
For example, in Hoenn all you could do was the Battle Frontier, and that was pretty much it. And in Ruby and Sapphire, there was no Battle Frontier. You could only look for the roaming legendary.
In X and Y, there is the Looker Bureau missions, the roaming legendaries, Zygarde and Mewtwo, Kiloude City (Friend Safari and Battle Maison) and possibly more I haven't listed.
In Diamond, Pearl and Platinum all you could do was go to the stupid Battle Area. And that was all. The Battle Tower was than happily replaced with the Battle Frontier, and the roaming legendary birds.
X and Y is probably much more interesting post-game, except for the games aforementioned that get places to visit afterwards. So, you should be grateful. x)
Hope I helped. :)