It's interesting though, that we may have to once-again use an elderscroll. What would be the purpose of it though? We know that we can look back through time using them, but why would we need that for Dawnguard?

Perhaps, we will look into the past and see the Snow Elves as they once were? Maybe their is another time-rift on the island of Solstheim on Jolgeirr Barrow where the Snow Prince is buried. It was featured in the Bloodmoon DLC, and you could also find the Spear of the Snow Prince in the tomb. Which means that they may be spears added in a future update/the Dawnguard DLC.

I've also found a bit on Dawnstar lore concerning the Falmer, which may be a possible DLC starting point.

"The garrison town of Dawnstar was once besieged by the glacier tribes, or 'Ice Tribes'. These were blue-skinned humanoids that had tamed trolls and the dreaded Gehenoth as war beasts, and had plotted with the traitorous champion of the town."

These are just quotes I've got off the wiki which I'm trying to patch together to get a good lore background on the possible DLC concerning Solstheim or Dawnstar, but admittedly I'm leaning more to Solsthiem, but it could concern both of them for I know.

Another beast in Bloodmoon (the DLC containing Solstheim) was the Riekling, who the Nords believed descended from the Falmer, I can't find much about them but a short sentence on the Bestiary page. "Rieklings are small creatures that appear in the winterly land of Solstheim. Despite their weak appearance, they offer quite a challenge, and carry their own variety of swords and shields. The native Nords believe they are descended from the Falmer." One thing I can't seem to understand is how the Falmer and Vampires connect in the DLC, I can't see any connection in the common lore. I don't know, perhaps it will give us some vampire features with the DLC pack but it's mainly focused on the Snow Prince storyline?

You don't have to read on from here, these are just some quotes from the wiki for some references.

"After the Night of Tears in the late Merethic Era, Ysgramor and his Five Hundred Companions launched a campaign to drive the elves from Skyrim. Ysgramor and his heirs succeeded, and by the second century of the First Era, during the reign of the Nordic king Harald, the last of the Falmer were driven from Skyrim to the island of Solstheim. The Falmer's civilization was obliterated at the Battle of the Moesring with the death of their leader, the Snow Prince. The remaining snow elves were scattered or slain, and were never heard from again."

"The Snow Prince was a Falmer of great fighting prowess who lived on the island of Solstheim. His kin had great respect for him, and hailed him as their savior in the Battle of the Moesring. The Snow Prince killed many Nords in the battle with his ice magic and his spear, but was struck down by Finna, a twelve year old girl, who threw her mother Jofrior's sword at the Prince after she had seen him slay her."