I think we can all agree that if one PC in a party became a fully-fledged vampire in most fantasy RPG settings, they would be OP beyond belief. But what if they were all vampires (and the template was nerfed)?
What I have in mind is a campaign where the players will make their characters as normal, but as soon as the game starts, I will simply say, "Alright, all of you open your eyes. You memory is extremely foggy, so you don't remember how you got here. The small stone room is lit by one single torch and the light flickers all over and around your pale skin. You all feel very hungry, but oddly enough, it doesn't feel as though normal food will satisfy you. The strangest thing of all, though, is that you can't feel your heart beating. Everyone add the vampire template to your characters."
Now understand that my intention is a nerfed template in the sense that you "unlock" some vampiric powers (from the template or homebrew) as you grow. The only things they'll probably start with are basic undead traits, Darkvision, +2 Strength, +2 Charisma, and the ability feed and create vampire spawn. As the vampaign (puns
) progresses, the PCs will choose whether to embrace their newfound vampirism or oppose it and loathingly live until they are killed or can find a cure.