There are rumours of another dlc called heath fire,so even if the snow elve's don't appear in dawnguard maybe they will appear in hearth fire if it turns out to be true of course.

Hearthfire is a pretty odd name.
What could it possibly mean?

Through out the game there have been hints about the snow elf return... mentioning of falmer attacking settlements and the loading screen discussing the history of falmer next to a picture of what i would assume is the snow elf prince.
my guess is that the snow elf return will be a whole other add-on

Call me crazy but what if the Vamp boss uses the Elder scroll to bring the snow elf prince back from the dead or at least into the present day and together they plan to put out the sun? Also if snow elves are a big part of Dawnguard I want to play as one afterwards....because screw lore! lol

Why dream? The answer was given in Morrowind, specifically the Mages's Guild questline. Basically, they're all dead. High Engineer (How cool is that job title?) Kagrenac used the Tools (Keening and Sunder) on the Heart of Lorkhan in a desperate attempt to turn the tide of the War of the First Council. Faced with the prospect of defeat, he risked (And lost) his entire people in an attempt to ascend the Dwemer to a level of power even greater than the Dunmer Tribunal.

There does remain the possibility they exist, but in a realm independent of Nirn, so for all intents and purposes, they are dead, with no hope of crossing the Liminal barrier into Nirn, should they have actually been transported.