It sounds to me, with these vampires trying to 'end the tyranny of the sun', like Namira might have a hand in this DLC. This is further given evidence by the dark and bleary realm we see the PC enter. It would make sense for this to be some kind of war between Namira and another Daedric Prince... perhaps Azura.

Speculations?

I hope there's no much of Daedra involvement.
I want more of the REAL ES WORLD , no more of these dimensions !

I actually see Vaermina having a hand in it, not Namira. After all, I see that she's the actual creator of vampires, not Molag Bal. Reason being, Molag Bal had to get the vampire cure from Vaermina, and his cultists deny that he made it. Instead, I think Vaermina created the disease, infected Molag Bal with it, and he spread it like a bad case of STDs.

Oblivion is part of the real ES world.

Actually, Namira sounds like a logical choice. Consider the Namira quest in Skyrim and its location. It fits rather well with what we've seen in the DG trailer.

This sounds pretty good to me.

It could be that both are involved in the story behind the vampires. It's pretty hard to think that the shrine in the trailer is anyone else's but Bal's.