A cool thing I was thinking, is a shop. A shop where the creatures could go spend their money, they earn on payday or perhaps "win". So the longer you have a creature in the dungeon, and if it hasn't spend its money, it can buy something big, like Trolls can buy bigger hammers, and bile demons getting more fancy stuff attached to their horns.
I was also thinking it could be cool if they could buy upgrades to their lair, so they will recover faster, or tools so they work faster in the workshop.
These things should be independent for each creature, so the player should not have any influence on the shopping. Also in order to add a bit of surprise and dissapointment, when you find out your biledemon just spend money on a fancy skirt or whatever.
I really like things in games where the player doesn't always have an influence. In my oppinion it will give the creatures and the game more character.