Something I've wanted to run would be a campaign with an elven fighter-mage running solo. Maybe with one NPC follower. Kind of like Elric and Moonglum. Start the character at level 10 or so, and work up from there.
Of course, I'd run it in the Forgotten Realms, probably the Western Heartlands. Travel from Evereska down to Iriaebor over to Candlekeep and back. Become a famous adventurer that wipes out bandit camps and defeats evil mages.
It seems kind of lame and boring, but I think it would be a blast. Especially if I focused a bit more on acquiring old magic and researching it.