I think we can already assume that Carmilla and Cornell are both Lords of Shadow. Compare the trailer with newest screenshots (you can check them out in CastleKeeper's Gallery if you want, just click the image in my signature) - there's always a pillar of blue light in the place where we meet them. What's most important - in the trailer we can see a fragment of the mask slowly falling from the sky in such "pillar". We also know from previews that Lords of Shadow themselves have those fragments...
So in the trailer we probably see their last moments - they give us some piece of information, fight with us, change into stronger form (I think Carmilla's one is this with long claws), die in epic way and a mask's fragment appears.
There's also this thread about control over humans' souls. Gabriel will probably gain a new power from every Lord of Shadow (we can see him gaining wings in the trailer - that reminds me of OoE, I loved those

). From trailer we know that those powers are "the key", so that's probably what Dracula or someone who will become Dracula wants Gabriel to do - gain them all and bring them to him.
Any other speculations?
