Uhm honestly I'm really excited to play this game, it's under my favorite genre from what I've read so far. But anyways I don't know the theme exactly (some say vampires) but is that's the case I just wanted to mention and suggest a few names of infamous historical figures that could and might be used as bosses here goes:

Countess Elizabeth Bathory: also know as the The Blood Countess
Her story is gruesome but hey it's supposed to be a DARK themed mmo

Vlad Dracul: a.k.a. Vlad the Impaler
Hello original "Dracula"

Gilles de Rais: infamous for...you should do some research...this one is crazy lol

Lilith: only thing is that the enchantress set currently in PL is called Lilith but she (a fallen angel) had a pretty interesting story.

And last Jack the Ripper

Let me know what you guys think!
I'd honestly love to see these guys/gals come to life in a game lol