All of you have seen the 'teaser' pic from Bethesda regarding the DLC - it's the morose looking nord dude that we've seen before, his horned helmet hiding most of his features.
The salient differences:
1. His eyes have a reddish-gold tint/hue.
2. His pupils appear to be yellow/gold.
3. There is a yellow-white/mostly white light behind him and a bit to the left.
4. He has a quiver of arrows on his back.

Regarding #1 and #2 - DID YOU SEE THOSE COLORS IN MORROWIND? If Yes, WHERE?

I only played oblivion and don't recall seeing colors of that sort.

The eyes were dawn I think. Sun light.

It is definatly the sun behind him, it seems he is fueled by the sun. Considering the title 'dawnguard' i'd say this is about some sort of order of guardians of the dawn. I doubt there is a Morrowind connection, more likely it will be about Skyrim's legends again.

Which is okay, both Morrowind's and Oblivion's first expansion dealt with something native (Tribunal, Knights of the Nine). Their second expansion was more outlandish (Bloodmoon, Shivering Isles).

They are taking their time to release this so I think we won't be seeing more than two 'true' expansions.

Dang. I was hoping it was a clue that would lead to something - like the yellowish color being the same as dwemer structures in Morrowind or something like that. Oh, well.

From Dusk Till Dawnguard => vampires? => Game Jam 2011