That would definitely change Vampires to become their own, scary faction. Also, battle events with vampires that could be filled with thralls (simple humans) who serve them. For now they are really… “meh”.
Definitely this would be good.
I could imagine those vampires to be better organised. Think of those normal vampires we have right now as lower class vampires. They are dangerous nonetheless and they are immortal. Would they throw away their eternal life so easily? I think not. Not before they have thrown all their ghouls (could think of them as a vampires war dog) and their thralls (humans like bandits but more of the cultist kind) at you.
Then I could imagine a stronger version of vampire (you already had a alpha version that never made it into the game). It should be a rare and dangerous foe. And I have an interesting idea for a special ability. Vampires are all about blood. So why not add a little bit of blood magic to this?
So if one of his thralls or YOUR mercs dies he might reanimate ONE target at a time to be a “blood zombie” (there is definitely a better name for it out there). So what would be the difference to the normal zombie – necromancer connection? There can always only be one blood zombie per ancient vampire at any given time. This one zombie is buffed to be stronger than usual zombies BUT also dies when the vampire dies.
"I am a Paladin!"
>OMG, Malthus, there are no damn paladins in Battle Brothers...<
"OK, OK! Then I´m a wrecked down minstrel drunkard pretending to be a paladin, singing so wrong in the midst of battle that even the undead run in fear... Better?!"