The Player made Maps is an excellent idea, it's not as big as "make your own dungeon" but I think it's doable without costing too much time to the dev team.
You could add a "capture" or "train" mobs to add it to the trap, but the reward should scale depending of it.
For exemple, you choose the reward first (that you must provide yourself), it gives you a number of point (more for better reward) and each mobs needs a number of point to be added to the trap.
Then you go bury it somewhere and get a bottle you'll put somewhere too (AH or added to the pool of player map that could pop randomly with any other maps).