You should check out Nehrim, then check out the time it took, the team size, and the money involed. This thread may aswell be deleted. It wont happen.
It depends on what you mean by completed, and when.
A lot of the problem with these kinds of mods is that they aren't organized, and bite off more than they can chew. I can actually see a route for doing the entire landmass of Tamriel in Skyrim, but it would take a lot of patience and discipline as well as work.
1) The first step is easy - take the current work from Oblivion, Unique Landscapes, and other landmass mods for Oblivion (including Morroblivion) and convert the LANDMASS ONLY into Skyrim's worldspace. This actually shouldn't take long, as it can be automated with tools, but it would need to be checked for errors. When you are done, you have a huge, dirt-colored continent.
2a) Use either Skyrim native, or third-party meshes and textures to repopulate the landmass. This could take a while, but we have the original games and mods to go by, and plenty of pictures. I'd use Skyrim creatures, simply because they are "legal", and probably improved as well. We know deer and elk are in, so that shouldn't be an issue. When you are done with this stage, you actually have a playable mod to release - though it would be a bit boring at first. More importantly, you have the stage set for the next step - actual game play.
Sure, it's really only useful for exploration and hunting, but at least you have the moral victory of achieving the whole landmass of Tamriel.
2b) Start putting in new dungeons, using only Skyrim resources. These are legal, and would allow for actual game play in the new mod. We could place all new weapons and other items in these dungeons. This could happen in parallel with #2, so long as space is left for important existing sites (like the cities from Oblivion and Morrowind)
3) Work on existing ruins and cities from the old games. They can either be the same as the old games, or all new ones. This is where the hard work comes in, and why its nice to have an existing mod release to build on. New meshes and textures will need to be made, new NPC's and quests written, etc. You could spend the entire life of Skyrim just building on this mod.
I would want to see (obviously) all of the cities from Morrowind, Oblivion, Daggerfall, etc. I particularly want to revisit Daggerfall - if only for old times sake. This *could* happen in parallel, but this is where people get bogged down, are tempted to use old resources, etc. It would be better if the landmass were more or less complete before starting this step.
It's doable with the right crew of modders, and a bit of discipline.