If the Black City IS Arlathan (and I doubt it, because we've seen some concept art that I'm pretty sure was meant to be Arlathan in-game for DAI), then it's likely that the gods are sealed away in there, and something to do with that--Andruil and/or Falon'din, most likely--has created the Blight (their anger/rage?). But it does not explain why the city stands empty.
I agree with a lot of this other than the last part -- I'm pretty convinced the ancient elves had nothing to do with the creation of the Blight, but were instead victims of it. The mosaics in the temple of Mythal clearly mention how Andruil was maddened by the Blight when she went to fight it. I know they got a lot of things wrong, but because its in this temple I'm inclined to believe it's closer to the truth.
I guess I'm just wondering how they are going to resolve the him being immortal and her being mortal issue should they stay together. Seems like they either have to make him a mortal or elevate her to god-hood.
I think either possibility could be introduced without breaking things. More likely Solas would lose his immortality like all other elves, but I could see it either way.
Re: Happy ending: It has to be happy, like, happy-happy as in we are reunited, work things out, and we get to live together. I'm so over bittwersweet romances, and I never like the idea of "being happy together in the afterlife", unless the afterlife = permanent exploration of the Fade together.
To each their own yes, let there be options to make it tragic or whatever but please, for the love of god give us a good one. The Alistair romance in DAO was just ugh choices, and while I liked Fenris, none of the DA2 romances really clicked with me anyway. Solas is seriously in his own league as far as romances go, so I would just really love an actual happy conclusion. Pretty please with kittens on top.