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.