Ooh, this is a really neat idea.  It sounds a lot like a vivarium game, which is an interesting genre.  Earthtongue is my favorite vivarium game so far, so maybe looking at the trailer could give you some ideas?

Here's a few ideas of my own:

• Slight procedural generation of the room layout
• There's a back-and-forth hero/monster relationship, if heroes are too strong, monsters will power up and vice versa
• Maybe sometimes a prompt will pop up and you can choose an option, like installing a trap or changing monster spawn rates or something
• You can drag around or guide the heroes or monsters to give one side an upper hand
• Maybe you have to choose whether to control heroes or monsters and have some incentive to make your side do better?  Or maybe sandbox nature would be more fun.
• Heroes or monsters can rank up after doing well?  Strong heroes may return to the dungeon after leaving.

But these are just spitball ideas.  I like babyjeans's ideas a lot too.

I want to play this now! I hope you make it!

I have been working for a while today and this is where I have gotten with my progress. This may look more impressive than it really is. It is in no way interactive yet, but I've got a slime cube moving around using a Unity Navigation Mesh. I made the torches with some particle systems and unity cylinders, and the cave floor is a Unity terrain with some stalagmite structures painted in. That little glowing wafer on the floor is a test of a gold coin.  To make the slime I used Blender and subdivided a cube object and stretched the bottom part a bit.

I am going to start working on allowing the player to feed coins to the slime.