Edit short version: Is it even possible to add a quest line that only happens, or only takes place after the main quest of Dawnguard, while still being compatible with nearly every mod? Or would such a thing conflict with nearly EVERYTHING?
I guess it's obvious that altering the MQ itself would cause compatibility issues with quite a few mods, although I wonder if there's some work around for just making quests that are only available AFTER the main quest? (Without mod conflicts).
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TLDR stuff below
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No I'm not saying I'm likely gonna do this, I don't have the skill, and likely don't have the resources or time to do such a thing. It's just something I thought, would've been a massive improvement... Don't get your hopes up about this, it's just a "THINK of the possibilities and potential!" type thing.
http://www.uesp.net/wiki/Daggerfall:Vampirism
http://www.uesp.net/wiki/Morrowind:Vampires
So imagine how epic Dawnguard would've been, if it had every bloodline from former TES games was a part of it. Anthotis, Garlythi, haarvenu, Khulari, Lyreiz, Montalion, Selenu, Thrafey, Vraseth, Quarra, Aundae, and Berne. And the storyline would go something like this, you could side with the Dawnguard or Volkihaar but the consequences of your decision would be far greater than they were in vanilla.
Your reasons for siding with the Dawnguard (disguised as a mortal) would be, you don't want your Volkihaar rivals becoming even more powerful, where as your reason for siding with the Volkihaar would be, cause you know, that whole sunlight thing, and how it's bad for vampires and all that.
And even fellow vampires of your bloodline, would have political conflicts amongst each other about what the right choice is, IE saying things like vampire A "I don't like the sunlight anymore than the Volkihaar do, but if we help them, they're gonna remain the dominant vampire clan here, and maybe even betray us afterwards and try to annihilate us", or B "We can worry about Volkihaar and the other bloodlines later, the Dawnguard is the greatest threat to us at the moment".
(Something that would even at times split up a single bloodline, like the Stormcloak/Imperial Legion conflict has split up families in Skyrim). Perhaps battle in an miniature arena run by vampires, and whoever wins these arena matches gets to decide the course of action, in these difficult decisions.
And even whole bloodlines would have be heavily divided over the right course of action. You'd never know who'd take your side of the matter, or who might attack you on sight, who might hate you for being of a different bloodline, or hate you because they don't agree with your views on how to handle the Volkihaar vs Dawnguard dilemma.
And then... At the very end, if you sided with Volkihaar, after the defeat of the Dawnguard, eventually the formerly allied bloodlines, would turn on each other... And now you had to battle against the rival vampire clans, especially the Volkihaar if you were anything else, in a quest to make your vampiric bloodline the dominant one. And then things would be kinda like that Wars of Skyrim overhaul, only many different factions of vampires slaying each other.
And perhaps as an additional quest line, you'd have to rebuild something like a gigantic Greater Flame Atronach (or some more unique type of lore friendly fiery creature that could be constructed), not only helping with fellow mortals, but also helping against most of the other rival vampires (the weakness to fire all vampires except for the Selenu have). You'd need lots of materials, a very powerful soul, and other things to create this creature... Who would act as a champion for your vampire clan. While I'd love to make it huge size size wise, that would simply be too taxing on the computer/cause severe lag /etc/, so it wouldn't be too big.
For balancing reasons- in addition to it's huge size, this artificially constructed champion of yours, would never leave the land where Dawnguard takes place, and you couldn't have it follow you around or summon it at will. It would be extremely hard to get the materials for it, and you'd have to journey all over the place to get them.
Perhaps you'd have to even refuel it with souls every now and then (just an idea). And you'd encounter heavy resistance, from other vampires who want to control this creature themselves, or fear how dangerous it could be to vampires if it went rogue or something, and the last remains of the dwindling Dawnguard, who don't want this thing they'd no doubt view as an abomination supporting your character they'd also see as an abomination.
Or if you sided with Dawnguard, you could manipulate them into unwittingly exterminating rival vampire clans, all while you find yours a safe place to hide, and choose to continue to live in stealth... And after the main quest either try to by stealth, overthrow the Dawnguard who've no doubt been weakened by the hardships of the main vanilla quest, and perhaps feel overconfident after a recent victory...
Or you could in a much more direct about way, oppose the Dawnguard openly, with no sinister sneaky plans... And with the Dawnguard having weakened much of your competition, you might be able to take them out, without the help of the Volkihaar, or any other bloodline you aren't a part of.
Perhaps, try to make an army to defeat the Dawnguard. An army made up partially of ruthless (and maybe somewhat foolish) greedy mortal mercenaries who'd sell their sword, bow, crossbow, or spells to anyone, even vampires, conjured creatures, and repurposed dwemer robots, because they don't have these weaknesses to sunlight, or silver weapons or fire. Volkihaar would start out vastly outnumbering other vampires, and generally being much more powerful in the land Dawnguard takes place in, for obvious reasons, I mean they are in their homeland and all.
So anyways... This is just an idea I had, nothing that's going to be very likely. But if such a thing was done well, scripted right, quests done right, balanced right, portrayed right, /etc/ and all that other stuff, just how epic of an optional quest would these sound like? How would have you felt if that was the path vanilla Dawnguard took?
And lastly... Just how difficult would it be, to make all these quests, scripts, and all that work for such an ambitious mod? And especially in a way, that would be compatible with almost all other vampiric or Dawnguard related mods?
(I'm saying this all, because THIS is how I wish the Dawnguard questline would have been, and I've sometimes had fantasies of trying to make this all real, even though I sadly feel I lack the time, skill, and all that required to make such a thing). (I know addspell/addperk potions, combined with vampire NPCs with unique factions, can solve the one dilemma of adding in multiple bloodlines, but a bunch of this other stuff is much more complicated).