Read Monte Cook's
Design Secrets: High Level Adventures to learn about the unique game elements that high level play will bring. It's a different game at high levels and you need to be ready for it. And this is the article that had it all click for me in my head, and I learned how to
stop worrying and love the bomb. Suddenly the old adventure plot structures fail to challenge anymore. No more wandering encounters. Forget about traps. No more using classical mystery/investigative plotlines that are easily adapted from books and TV shows. Storming a castle is just a Scry and a Teleport away. So, be ready for it. Along with the article that I mentioned, the Epic Level Handbook has some good adventure design advice that applies well to the "teen" levels.
With high level gaming, I like to think BIG! The entire multiverse is the "dungeon," teleport/wind walk/plane shift is the "hallway," the target location is the "room," and if that location happens to be secured against direct entry, then the surrounding structure and wards is the "door" & "walls."
The fact is that 20th level adventurers can accomplish certain adventures in a day or two that would take a 15th level party a month to do, and would be impossible for a 10th level party to do. Each "dungeon room" for a 20th level party could have been the entire "dungeon" for a lower level party. And each relevant adventure site could be spread out enormous distances, including between planes, but it's just a Gate and a Teleport away. They just need to know where to go. And that's the key. They don't know where to go until they discover that a location (or creature or item) is relevant. You can string several locations together with each one revealing the relevance of the next one. If you flowchart it, you got your "dungeon map." Yeah, the party can buff/scry/teleport to the BBEG, but they can only puzzle out his true identity and where he located at the next-to-last location of the adventure. Or maybe they can only pin him down and keep him from escaping at the last location. A BBEG that keeps teleporting away before the party reaches its initative is a high-level problem to be solved.
I like to find ways to create scenarios that represent the unique aspects of high level play. Setting them in very hostile environments, supernatural locations, things like that. I like to find ways to make it distinctive from lower level games. Setting up adventures that would be impossible for a lower level party to do. My Epic players aren't going to walk down the forest path, face some wandering monsters, then clean out the abandoned castle. They are going to teleport to the bottom of the ocean thousands of miles away, then swim through a vocano on the other side of the world, then plane shift to the tunnels of insanity on Pandemonium to gather information from an imprisoned god, all within one gaming session. Plus they have to deal with teleporting ambushers during their downtime, like the characters of Charmed have to (I find Charmed to be a good resource to use for designing high level games; the latest Stargate SG 1 seasons is another good example of high-level play.)
In addition, information can arrive to the high-level PCs at an incredible rate. Information that they would have had to quest for in the past is available to them right now if they have the skills or spells to get it. If not, they only have to Teleport to the known expert of such matters and persuade him/her/it/cthulhu to reveal their knowledge.
When you create high level adventures, create legendary myths.
