Going to the big leagues
Our gaming group has two campaigns on the border of Epic. For our groups, our DM's put our heads together to brainstorm things that would be good "long-term" threats that were behind the many plots in our combined campaigns. Our hope is to "open the doors" to Epic for both campaigns, and more...
As we did our brainstorming, we realized that there were many heroic campaigns from 1st and 2nd ed that people might like to play, so we're now working on a way to combine these older campaigns into the "history" of our world.
The relevant issues seem to be that "Epic" challenges typically have an "earth shattering" impact, so it's best to begin with the end in mind, and identify several major threats that could be quickly developed or uncovered by your parties. At the same time, it's important to keep in mind that these threats haven't taken over the world to this point...so why not?
We've not made the transition into Epic (ie, played the 20th level module), but I'll most likely use the same format of story that I've always used, except instead of "dugeon-delving" they'll be have to be more challenges like "saving the village" and "protecting the innocents" ala comic book superheroes.
Good luck with your campaign.