I've always been jealous of DMs who could run multiple campaigns in a single homebrew world, but as for me, I'm a one-shot guy. I tailor the world specifically to a campaign. Usually the campaign evolves into a save the world campiagn and it feels used up after that.
As an example, my current homebrew was designed specifically to support Gestalt Characters as well as non gestalt (so I didn't need to hand create every NPC/Monster) The theme of the campaign has been the characters in the wrong time. They have moved through various distant past and distant future scenes. I've been tempted to run further adventures in this world, but Once they've saved this world, they could move on, but since they are heavily armed with WWII era weapons, it wouldn't be two much fun. My cosmology aslo limits future adventures.
A new party would be a possibility but since the players know many of the secrets of the world, It wouldn't be as much fun.