While I love the idea of making our own dungeons. It would have to be rewardless unless adopted by the devs and tuned to the appropriate difficulty.
Imagine people deliberately making easy dungeons to be repeatedly farmed for a currency or gear reward.
Maybe difficulty settings with a required number of “points” would help curb this. Each mob type is awarded a certain number of points that increases based on proximity or aggro radius of other mobs. Placing the mobs into the dungeon would “spend” these points. The targeted difficulty would increase the minimum and maximum points allowed for the player created dungeon. Even then people will find the easiest combinations of mobs and make easy-to-farm dungeons.
I’m sure there is a way to introduce this concept and still provide rewards, though.