For some reason as soon as you said this I though of "The World's Largest Dungeon" from DnD 3.0 (for those who don't know what it is it is a dungeon that takes a group of 1st lvl characters all the way to lvl 20 in a single dungeon).
But I do like the idea but I would say have a couple of them instead of a single one or have the different dif. settings be around the following, Casual: 1-5 Normal: 6-10, Hard 11-15, and Elite being 16-20 simular to how Devil Assult is.
Just my thoughts take them at face value and nothing more.