Mmm, this definately is one of those things that I would love to be a part of.
If I had like three more lifetimes worth of free time that is... or if some crazy rich man or woman would pay me to do it.
Even if someone manages to create an automated object export/import program, pretty much nothing would end up placed properly. So we'd have to go in and change everything by hand anyway. And that would take ages.
When that's done, we'd need to place all the doors leading to existing interiors (using existing interiors would at least cut the total required time in half). The cities would definately need overhauls, as the larger world would make the current cities look like simple fortresses. It's still
possible to just leave them and the current city worldspaces as they are though.
Then we have to create pathgrids (I don't know how efficient the "automated" pathgrid placement is, but I'm guessing - Not very.
Then when that's done, we'd need to change every single AI package for the NPCs and place them at their "new" locations (unless they start in interiors).
After that, marker placement time - Every important marker in the game needs to be in the new worldspace.
And finally, when all references are placed and the world is setup. We need to check all quests and all scripts and update them so they point to the right references and worldspaces.
How many years did it take for Bethesda to create Oblivion?
Damn, I'm sorry, I'm pretty negative about this. I still absolutely love the idea. But I just can't see this being done by people not getting paid
For anyone still interested in the massive world idea, isn't Mesogea (or some name like that) still up and running as a WIP? The differencein here would be that we wouldn't need to create new resources, we would just need to copy everything. So with that in mind, maybe this is more actually plausible that that one...