If I ever hated myself enough to DM, I always thought a short-ish campaign based around amnesia would be fun. The players can pick their race and class, but they have to discover their background, personality, and what they're supposed to be doing. I imagine it would be tricky to run though.
The other one I would want to do is a campaign with an initial employer who ends up serving as a sort of anti-villain. A Paladin, part of a group of adventurers who came to a city state to try to (semi-)peacefully end a civil war. One by one his party was picked off by the violence, until only he remained, at which point he abandoned his good and holy principles, starting playing dirty, and ended up ruling the city-state with an iron fist. And as oppressive as it is, it's still far better than it was before. The new justice is harsh, but at least it's not mob justice. But both parties of the civil war are still lurking on the sidelines, and neither of them like the new king...