The only issue I have with this idea, is that it’s too MMORPG, and not Sandbox Creative.
Instead of A, using Portals, and B, having the Devs make it random. How about the PLAYERS build the Dungeon Areas?
Not just Dungeons, but other Challenages too. Puzzles, Jumping/Platforming, Finding Secrets, Mazes. Not just have it be Combat Focus, but CREAVTIVE Focus based on many designs. And have the Creators of the Dungeons fund the rewards, or have the rewards based off different factors.
City of Heroes, for example, allowed players to build their own quests and levels for the players. Allowing them to pick the look, how it’s shaped and design, how complicatedly long or simple and short it was, what enemies spawned, how high or low level the enemies were, even what skills and abilities the mid bosses and final bosses had. And at the end, had the reward based on all those factors. How long it was, how many enemies, their level compared to yours, and so on.