well, the trailer says things like "they've returned", so since there was an akaviri invasion in 2E or something, I assumed it would be them
they wouldn't really call fighting Alduin a 'war'
It doesn't have to be Akavir, it could be any group that has previously been defeated but is now returning, and in fact Akavir seems unlikely to me, seeing as there was also a bit about how people didn't want to believe they existed, and it seems to me that the existence of Akavir has never really been disputed. For this same reason, I also really don't by into the notion that whoever is returning is dragons. I suspect we might be dealing with a more obscure threat, though what, I can't really say, or a previously unknown one, but we really won't know for certain until Bethesda releases some information on the subject. I wouldn't be surprised if its in the Game Informer article, since Bethesda never tried to keep the whole Daedric invasion in Oblivion a secret. Although I suppose they COULD decide that they want the nature of Skyrim's antagonists to be a surprise. I'd rather not speculate too much on the subject now as there's a good chance any theories we reach will be proven wrong.
My personal theory is that the devs said to themselves "Gee, I'd like to do a quest in Oblivion dealing with the Akaviri, but I can't go and make snakeman models for just one quest... ah, screw it, I'll just use humans. Who'll know?"
That's the theory that I assume is the case to, because let's face it, making completely unique snake-men models that will only be used in one quest would kind of be a waste. It might have been worthwhile for part of the main quest or even a fairly major guild quest, but we're talking about a side quest some players might never end up doing here. I'd think of we were meant to make anything of it, they'd give a bit of information on the subject, the way its handled in the game, it seems we're supposed to just go with it.