At this point it's a matter of scope, with 20th level characters essentially moving to the biggest leagues. Much like superheroes, these guys will be dealing with cosmic scale stuff, even if they're schmucks like Guy Gardner (a Green Lantern w/ emotional & social issues).
The PCs have gone from having minimal effect upon the world and world (environment, obstacles, etc.) being significant, yet they also have more powerful people around them. Now it's the opposite, which often means having to upgrade the environment and cut off the heroes. Ex. The end of the Rise of the Runelords AP goes to where breathing's difficult and there are dimensional barriers making teleporting in or out unreliable.
So rather than "while you were guarding this caravan you happen upon a clue of a threat to the local village", you get "while you were guarding this metropolis from a threat too big for their best defenders, you happen upon a clue of a threat to the region, world, stability of space-time".
(Some APs plant a large seed early, in preparation for the endgame. Speaking of which, perhaps all include a section on what to do post-AP, and those are full of story seeds for highest-level play.)
Old issues of Dungeon magazine have a few labeled Epic. They most often take the PCs to alternate dimensions or involve wannabe gods or destroyers of the world. Golarion's pretty good at having several areas which spike in level, like Treerazer's abode or where ancient evil rulers still sit upon the throne. They may only be about the players' level, yet would also have lots of resources.