I really hope there are multiple playable vampire bloodlines... I'll still play either way, but this very simple thing would vastly improve the game for me. Now under IDEAL conditions, there would be NPC vampires who would only be friendly towards you if you were of their bloodline. This could be done by simply making it where they won't attack the player in question anymore, so that player couldn't like harm their own faction by assisting these vampires attacking players from their own faction, but they wouldn't be attached by them- and have a minor benefit from such... And it'd add lots of roleplay value. That's only ideally though... Still, it'd be relatively easy to simply implement something, where players can only choose to be a part of one specific bloodline, and at the very least, they get permanent stat bonuses. The stat bonuses would being stronger/faster/tougher, and slightly better at everything than a non-vampire version of themselves would be... And then depending on their bloodline, excelling to an extra degree at a specific thing.
At the VERY VERY least, they could at least make a defacto warrior vampire bloodline, stealthy vampire bloodline, and mage vampire bloodline, and while all three of them would be slightly better at everything than non-vampire equivalents of themselves, the warrior bloodline would have a significant amount of extra health and damage mitigation and a passive buff to melee, the stealthy one would have a significant amount of extra stamina and speed with extra bonus damage for stealth attacks, and the mage vampire bloodline would have higher magicka and the most powerful health drain ability along with a passive increase to magical abilities.
In order to choose the right bloodline, it would be relatively simple to set up, simply a quest that involves tracking down blood potions, and you can only choose to drink one, so you drink the blood potion of whatever bloodline you want to join. And I know, for a lot of people, simply customizing their character via perks is all they need to feel content, but for me, and some other players, it adds in a whole new level of immersion, when what bloodline you choose, is the primary deciding difference between you and other vampires.
It would be especially interesting, if there were some player created all vampire guilds ingame that only accept players of the same vampiric bloodline into their guild... So even though you wouldn't actually be fighting against other players directly based on their bloodline, there'd still be somewhat of a sense of tension even between differing bloodlines, even though they fight against common enemies. I can imagine in this war torn scenerio as a lore based explanation, that say bloodline A might hypothetically after the war ingame is all over might plan to attempt to exterminate bloodline B, but for now they'll tolerate each others existence until the war ends... Just they wouldn't accept each other as guildmates or whatever, and there'd be a certain level of coldness and animosity between them.
That way it'd somewhat make sense, wouldn't be too hard on the developers in theory, and the gameplay wouldn't have to be radically changed for this to be implemented. It'd still gameplay wise be a war between three factions, but there'd still be a certain level of animosity and internal strife adding roleplay value.
Or perhaps more interestingly, if different factions had different bloodlines, then to some degree, there really would be different bloodlines actually fighting each other, although if that was done, they'd somehow have to balance them to be equally powerful in spite of differences, which could be difficult.
That all said, if they just do so much as make different bloodlines an option, it will vastly improve the game to me.