I love the idea, and I fully support it. The tough part will be finding ways to properly reward players without allowing them to farm the dungeon. A chest full of loot is fine, but you'd have to refill it - and what would stop that one player from going through again and again?
Also, the really fun part will be finding suitable places in the world to place the portals. I suppose they'll fit to the theme of the dungeon... a nether-y dungeon could have a portal just laying in the open far away somewhere, surrounded by netherrack and soulsand, etc.
Anyway, keep it up!