So will the HD DLC get updated for Dawnguard? Or will there be a separate Dawnguard HD DLC?

Just wondering.

Dawnguard mainly use textures from the main game, so I'm not quite sure a Dawnguard HD texture DLC is needed.

Well all the new armor, weapons, monsters seemed to have new textures unique to Dawnguard. Maybe it wouldn't need to be a large patch/DLC (like the last one) but there are several things that could use the upgrade.