Wake in Another World
When an adventuring party sleeps they awake in a different dungeon. It is a real place, not a dream. When they sleep there they awake back in their first world. How long does this go on? What caused it? What interesting things can be done with alternating one-day time limits?
Next level of the idea is that when there is a TPK the party wakes up somewhere else, another world, and they are changed.
You are a statue. You start play with ~5 HD and all the related bonuses. You never go up in level, however. And you do not heal. I wonder if balancing power with a limited total amount of life would be an interesting challenge for experienced players.
Option one is to allow "upgrades" by purchasing enchantments and smithing/stone work. Essentially you could "level" by using gp as xp.
Class as Race
Entering a profession in this weird world changes you physically. Wizards gradually become wizened, sexless things. Fighters grow in size and brawn up. Rogues become thinner and harder to see in shadows. Changing class might require surgery or at least prolonged sickness.
Class as Lifestage
In this weird world everyone starts as a rogue. When they die, they transform into fighters. And when they die in that form they transform into wizards. The order can vary according to your taste, but all are stages in the life arc of one creature. What will the final death reveal?