[@Cyclone] I'd say get the bones of a system in place first, but then ask for input to flesh it out. If the system's too heavily developed it would make ironing out problems more difficult, as you'd start to pressure other aspects of the rules wth each change.

Would you have a developing dungeon? For instance, the Dungeon Keeper starts with a gang of imps who he has to order to mine, build and dig, but as the dungeon expands the GM would be able to 'promote' the good players into different creatures as new rooms were developed and upgraded?

And then there's captured Fairies being tortured and converted, along with other heroes as well.

I'd recommend some sort of reward system for players who carry out their tasks quickly and efficiently, or at least do what the Dungeon Keeper tells them to do timeously. The players with the highest rewards would be the ones most likely to be promoted, whereas the lower ranking players would be sent on [s]suicide missions[/s] special assignments to gain points quickly.