Vanilla players complete dawnguard and simply forget about the dawnguard order that posses as big as a threat as imperials to a stormcloak. But nothing is done about it.
That's stupid - someone needs to extend the story. This would have to include two options for Vampires and Dawnguard players.
A vampires motive after defeating lord Harkon would be to either:
a) End his/her life as a vampire and revert to human (something a bit more emotive, epic & dramatic than the Vanilla method)
b) Continue the vampire regime by taking control into the dragonborn's own hands.
Option B would mean to completely wipe out the dawnguard - full, all out, vampire invasion of skyrim. As follows:
Killing any vampire that opposes the players leadership
Killing or turning various human NPC's throughout skyrim that have knowledge of vampire culture, history, needs, weakness etc.
Calling to all vampires, sliver hand, brotherhood, etc to report to Volkahir and swear their allegiance or die
Assassinating jarls, public figures, imperial/stormcloak leaders, the companions. Taking control of guilds, factions, and HALF OR LESS of towns & cities.
Half or Less locational control or Skyrim because the dawnguard will also be advancing through skyrim protecting holds, recruiting people etc. This means, for exmple, all South/Eastern cities (closer to dawnguard castle) will have x3 guards, many of which will be dawnguard and may question the vampire dragonborns reason for visiting (as a result of suspicion and vampire-identifying skills)
And in the case of a Dawnguard Playing Dragonborn, they'd basically be doing the same but less domination and more "protection" of holds. Either player-side will result in a vampire/dawnguard war similar to the civil war but much more gothic, stealth influenced and better (vanilla civil war is poooor).
Additional effects of the mod should also change the gameplay, although perhaps these should be optional to the player when installing:
modification of sunlight hours. vampire players can reduce sunlight hours to a minimum of 4 (like when closer to N/S poles).
Sunlight has to actually hit the vampire player directly for effect to occur.
half health, no stamina, no magica when in shade for first 1/2 hour of technical "daylight".
After first 1/2 hour of daylight vampire player will begin to smoke and eventually burn (perhaps over 1 minute real-time)
a burning in sunlight effect similar to dragon death with lots of ash and smoke and screaming, chared body parts - etc.
Dawngurd players can reduce night hours to 4, although the more night-hours the more vampire action.
vampire NPCs never attack vampire player on sight unless part of a vanilla mission or particular circumstances.
dawnguard do not attack werewolves on sight unless in particular circumstances.
vamp weakness to onions
vamp weakness to wolves
vamp weakness to divines
Just a vague idea i came up with (mostly while writing this!) but it would be great fun for any dawnguard DLC owner without effecting the lore of the game too much.
Upon completion as a vampire, the game would run fairly normally, except thier would be a unspoken, secretive yet well known vampire order running Skyrim from behind the Jarl's Chair (so to speak), the jarls would be replaced, constantly seduced, turned etc.
Upon completion as a Dawnguard soldier, Skyrim will go back to normal with only a small population of vampires scattered and living in the wilderness etc.
I hope people would like this.
thanks for reading,