Rather than try to retrofit the existing dungeons to be explorable, which is difficult to make open world anyway due to the massive changes made between the Normal and Hard Modes, why not create something entirely new?

I think the random generator would be the better way of making it work. However, I'd go one step further. Perhaps Neverwinter's Foundry system could be investigated and elements of it applied here to our Open World. Mainly just the delivery system by which players can accept and undertake the player made quests.

You could have a DoLand class, perhaps "Surveyor" that gains EXP by searching the world for areas to explore through "clue nodes". Once the Surveyor has discovered a location to be explored, the system generates a fully random (in theme with the zone its found in) zone to explore, populated with mobs similar to those found in the area also. There would be several different archetypes, such as Monster Lairs, Cave Systems, Old Ruins, Hideouts etc.

Once the the entrance has been discovered, the Surveyor who found it can create a Map to the location, which can be sold on the Markets just like the Treasure Maps, and accessed via the system used for Housing and Waking Sands. The Explorable zone could work much like the Dungeon System in the .//Hack PS2 games, whereby you have several floors of baddies with a chest at the end containing random goodies appropriate to the level of the Surveyor who found the place. The number of floors could even be randomized, so you'd never know just how deep it went, until you got the end.

Perhaps they can even have these Zones that are tailored to certain Jobs. For example, a Surveyor discoveres a mineral rich cave system, and a Miner buys that map, gathers some Miner friends (and maybe some DoWs for protection :P) and delves into the cave to hunt for materials. These areas could be used to drop some of the rarer materials, like the Light Steel and whatnot.
For Botanists, the area could be a hidden location of a forest, or oasis that yields rare plant materials.

I'd love to see a system like that. Maybe even allow these random generated zones to be found by multiple surveyors, whereby if several players buy the map for the same place, they'd end up running into each other.