|Caladyn Goblin Squad Member|
I think you'd be more interested in a product like the Neverwinter Nights series, the Diablo series, or the upcoming Torchlight II. I wouldn't be surprised if the NVN communities have unofficially modded the games to include elements/lore from the Pathfinder universe.
Some MMOs do allow people to create their own dungeons (i.e. Star Trek Online, Everquest II, City of Heroes, and the upcoming Neverwinter Nights Online, etc.) that may be used with friends, I believe. But this represents only a fraction of their respective game worlds.
I think it'd be difficult to capture the sandbox, one-world concept that seems to be the crux of what GW is trying to accomplish, while also providing this instance-heavy feature.