Simple idea: Add a new, rare and valuable troll type that only spawns on lower cave levels, to make mining on lower cave levels more rewarding.
Obviously, a gem troll would be a troll with gems studding their back and shoulders. When skinned, it gives several gems, perhaps of a large size. Only problem I see with this is that the gems will have crazy stats because trolls naturally have high quality. But given the sheer amount of gems I've seen people with, it wouldn't be too game breaking.