Ur (God Thought Experiment)
The core focus of Ur are "deals". When you make a deal with Ur, you get a permanent (Good) mutation; at the cost of something (generally current level, skill points or a negative associated with the mutation.) The positive mutations you get from Ur are permanent; even if you leave the god; as are the costs paid. In addition to the immediate cost paid; every deal you take increases your piety with Ur, which will decay slowly on kills only. Higher Ur piety makes enemies in the dungeon harder; worth more XP, but harder. You cannot leave Ur, unless you have no piety with him.
[That's literally a thought. I'm not really expanding on it right now.]