Those of us who've seen it have been enraptured by the mysteries of this peculiar place. What is it for? Why is it there? What role will it play? It suddenly appeared, one patch, and then was open in another. What purpose could this serve?

I would posit this: It's for the upcoming Thieves Guild update.

There are a few things that make me believe that this is so, and I will cover them for you.

1.) There are paths leading around the mansion that look like they would be used for guard patrol routes.
2.) There's a way into the manor via climbing on the environment, avoiding the 'main entrance' doorways.
3.) There's also a way back out, too. In a game that doesn't really have jumping puzzles, one would see this as suspect.
4.) There's a 'secret entrance' to the manor by way of the basement, which leads inside (a way around the guards?).
5.) The inside of the manor is more lavish than the vast majority of homes in Auridon, and there's much more to take.
6.) The manor exists as a compound, sealed off from civilisation. Like a beach home retreat for a shady character.

I do not know if I am right, but still... like you, I have been taken in by the story of this place. It has its own mythos, now. It has its history, its presence, and its allure. It is difficult to not think or talk about it. So I have been, with my beau. And we're confident enough that this is the future of Molemir's manor. Molemir will be a rich politician/merchant involved in one of your earlier Thieves Guild jobs.

I appreciate what ZOS has done with this place, and how much it's gotten my beau and I thinking about it. How well designed it is and fitting for its purpose, how much of a story it tells without there being any area relevant books or non-player characters present. I find I'm anticipating the Thieves Guild update now more than ever. I may be wrong, of course, that's entirely likely. It may serve some other purpose entirely and I'd have egg on me face, but still... I've a hunch that this is what it's for. And if it is, then I can see good things in the future. Very good things.

So congratulations are in order in advance, I think, to ZOS. And I appreciate that they allowed us to take a sneak peek at it.