We knew long ago that, if there was another game put out roughly on this schedule, it was going to have to be playable on the current generation of consoles. That meant that it was going to use roughly the same graphics engine. There would certainly be improvements, but there couldn't be a huge leap in graphics quality. That ruled Valenwood and Black Marsh out right off the bat - huge forests and swamp, respectively, would mean much more overhead for the background graphics, leaving that much less room for improvements in characters and animations. Elsweyr was possible graphically, but would require dealing with all the races, which would be an enormous development chore. Morrowind, Hammerfell and High Rock have already been done. Akavir was theoretically possible, but if they ever set a game in Akavir (and honestly, I sort of hope they don't), it's going to have to be very impressive, which implies both much improved graphics and much development poured into the races, setting, history, etc. So that pretty much left Summerset Isles and Skyrim.
I thought Summerset Isles was the more likely choice at first, simply because Oblivion makes notable mention of the goings-on there. I considered that to be foreshadowing. I visualized it something roughly akin to Camoran's Paradise - maybe a bit less rugged, but the same combination of nature and old, graceful little temples and walkways and such. Very cultured and very clean, with stately Altmer gliding along marble paths past fountains and gazebos. Doable with the current graphics and, again, strongly hinted at in Oblivion. But, even in that quick description, potentially at least a bit dull for those with shortened attention spans. And then the tide on the speculation board here decidedly turned toward Skyrim and I considered that. Graphically, a combination of the mountains from Oblivion and Solstheim from Bloodmoon, with valleys between the mountains, which could be fairly similar to the valleys in Oblivion - Nordic barbarians in fur armor with brutish weapons - no need to deal with any new races (Summerset Isles would've probably had to have included the Sloads, which would be kind of cool, but would require additional development) - and the forum was roundly in favor of the idea. It all made sense from a purely practical perspective, so I presumed then that it would be Skyrim. Then Beth reserved the rights to the name Skyrim and it was pretty much a done deal.
That must've been.... about a year and a half ago.