Well... for the island of the castle, I'd say... Olenveld or Roscrea. Same for some of the places we have seen. The frost giant is reminiscent of Solstheim, so he could live on another northern Island.
For the other realm... Well, it looks like Coldharbour to me. Moreover, Molag Bal is the creator of Vampires. However, two other hypothesis have been put forward.
First, those vampires ... well, don't look like the average TES vampire. I may be wrong, but so far, vampires had nothing to do with bats in TES.
So maybe they are some kind of "werebats" servicing Namira, hence this mould be Namira's realm. Moreover, in Oblivion, Namira gave the player a spell called "Namira's shroud" used to extinguish the light brought by priests of Arkay to a dark Ayleid ruin... And it seems that the "werebats" want to extinguish light at a much, much larger scale.
Another hypothesis I heard is that the strange deathly realm shown in the trailer could be the Soul Cairn, which is some independant territory of Oblivion, outside the jurisdiction of the various Princes, and ruled by some unknown "race", the members of which call themselves "the Ideal Masters". Could those, err vampires, be in fact the Ideal Masters?
Also, could these beasts be... Morpho?d Daedra?
Namira is a very good theory also.