In a future DLC (not necessarily Dawnguard) I'd love to see the rebirth of the evil Dragon Cult.
There's Dark Mages and Necromancers all over Skyrim after all, I could easily see them being tempted by a Powerful Dragon promising them power if they help subjugate Tamriel to Dragon rulership.
This plot could potentially offer:
New enemies
-- Living breathing Dragonpriests.
-- Evil Dragon Warriors.
-- NEW DRAGON TYPES!
-- Juvenile dragons that could be found in dungeon interiors.
New factions
-- Dragon Cult (totally evil): They worship Dragons, and want to help the dragons achieve complete dominion.
-- Dragon allies (neutral and good): They want Dragons to live peacefully alongside Men and Elves.
-- Dragon slayers (neutral and evil): The neutral members simply want to drive dragons away from Tamriel, the evil ones want an outright Dragon genocide.
Misc.
-- New Dragon NPCs
-- New weapons, apparel, shouts, and spells (duh)