One thing you could do is, once you treasure room is full then gold is automatically converterd into prestige items. These still count as gold but in gameplay terms they expensive looking objects placed around the dungeon. It could be a suit of armor place in hallway, a gold candle stick on a wall instead of a torch, etc.. Prestige items can be stolen by theives easily but they also are important for attacting minions. You could have it so that certain levels of prestige are required before you can access higher level minions, and options.

After all what kind of evil overlord doesn't have a few statues of themselves, or antique suits of armor in their fortress of doom?