1. The Chosen Ones.
I've always wanted to run a campaign where the Gods are dying, but they've decided to grant their powers to worthy mortals before they die. So, they send out emissaries to a group of people, (IE, the God Of War seeks out all the great warriors) and tell them if they can complete X series of tasks, they will become a God.
You could run this campaign for any level of characters, though I'm pretty sure it would wrap up around level 40, and the varied conflicts and quests you could integrate would be wonderful, in my mind.
2. The Dark Realms.
I really like the Realms, but it seems a bit too stable and non threatening at times, to me. So I've always considered making it darker, killing off Elminster and other protectors, and really throwing the doors open to the forces of evil that exist in the Realms to start taking over.
The only reason I haven't tried either of these campaigns is they would both take a lot of work and time to set up, which I don't have.