Thank you guys. Seems like they did that with Grimrock - the game even has very nice visuals (except the object shadows).
Multiple height levels, bridges and the like sound good. As Joozey said, a new game would need a new big feature/ twist on the used gameplay formula, but right now I can't think of any (except write a decent water simulation and make the player be a creature that loves to move inside water (corridors)).
If anyone has an idea, feel free to post it.