No one would do it.
Well, except people like me who enjoy dungeons for the experience of running the dungeon with friends.
What you are forgetting is that 99% of MMO playerbases will not do a damned thing if there is no shiny sparkly attractive reward at the end. And if the thing is HARD and doesn't give you the very best loot? Good luck convincing people to do it.
Do you really think people do Coil for "fun" and because its "difficult"? Nope. They do it because prestige and that thats where the shiny loot comes from.
Thats not to say i wouldn't like such a thing. Idea:
Create an "Adventure" Dungeon.
This dungeon would be procedurally generated, populated with monsters that match the tileset. For example, you get an ancient ruins tileset, the mobs would be tonberries, ghosts and skeletons and forests would have Ochu's and various animals.
Bosses can be found by exploring the procedurally generated dungeon.
These bosses would be large versions of a random mob that matches the tileset of the dungeon.
The bosses would have the same abilities of the mob it appears as, So if it was an Ochu, it would have Ochu abilities.
kill the floor boss to get to a new floor.
The difficulty of the dungeon begins at say, ilvl50. There are infinite floors, each floor generates a new map based off the Main Tileset, and then a few Subsets, like a Forest Main Tileset would have Cave or Ancient Ruins subsets that it could generate a 'floor' with.
Each floor gains difficulty, scaling upwards infinitely as you continue through the floors.
At higher difficulty levels, the Boss monsters would also gain randomly selected abilities from Raid Bosses found in normal Raid Content. It'd select from a pool of abilities that don't require special mechanics to avoid, For example you might get a bird type boss that has Twintanias Divebombs.
The rewards for this kind of content could be Gil, 'Aetherial' gear with a scaling ilvl, such that if you got to a floor that was as difficult as SCOB for example, the Aetherial gear that drops in that floor would be of an ilvl similar to that of SCOB. maybe -5 or something so it cant become 'BiS must have' gear.
The random nature of such a system would mean that it'd never be the same twice.
Perhaps even have it scale to group size, so you could take just 2 people or a full alliance of 24 if you wanted to. Or even go it solo and see how far you get.
And now its 6am and i need to sleep. Apologies if thats kind .. incoherent >_>