I saw that in the trailer, the player was killing falmer an in a falmer base. What would the falmer have to do with the quest? The clip of the player riding arvak seemed to be dawnguard - hence the crossbow and armor. Is arvak for both sides?

I saw In a trailer the Vampire Lord rampaging through Riverwood, that doesn't mean Riverwood has anything to do with the DLC. Rumor has it you meet the undead spirit of the snow prince as part of a side quest which would certainly involve Falmer.

Toddy also said there's a amazing side quest with a amazing dunegon that has nothing to do with the dawnguard main quest, it cold involve the falmer.

Aye. Like that weird giant thing (don't know its name), that doesn't seem totally plot-related.

Yeah but that was the gameplay. Im talking about the actual trailer...notice they add the new features and such, but of all creatures they chose falmer. Im guessing that yeah its probably a major side quest

"Major side quest"? I didn't know those words could make sense together XD

Major meaning a big [censored] side quest

I think Avark should/will be for both sides, becuase I personlly think Dawnguard will go into the Soul Carin to retrive the Elder Scroll, like the Vampires, but to hide/defend it from Harkon.