Thinking about Vampire hunting.
How about a hunter faction that deals with all things like Vampires, Werewolves etc?
Factions for vampires are a must. I'd like to see one for werecreatures too but not sure if it's possible in the lore.
Regarding hunting, for that, we also need clues and tracks and such. Otherwise all you're doing is wandering aimlessly until the right thing spawns, which isn't exactly "hunting". This is another area where good AI, and NPC behaviors going on by themselves behind the scenes, is vital. A hunter might linger in town and watch people, keeping track of those who never leave their house during the day, or read some weekly events report from the guard station about assaults, and see if any match the signs of a werewolf attack. I remember similar in Thief 3, a regularly updated bulletin on pickpockets/robberies/assaults/murders in the area. It wouldn't be the same of course, but if NPC's are capable of being attacked, it wouldn't be hard to have some weekly record on what happens to who. If you've got an "alive" world where bodies are moved and buried instead of hanging around for eternity, you could visit a graveyard to try and see a body before it'd buried. To run it down: join some hunting faction for authority, use authority to read the guard reports. See that someone died from slashing wounds. Investigate body. If it's mostly intact it was probably a smaller animal, but if it's dismembered and seems to have been a vicious attack, things are looking more suspicious. Leave the town gate at night, set out some kind of bait, wait. As usual the key is to build a living world, and then you can "hijack" the daily actions that are already going on regardless to add extra content like this with minimal effort.
Vampire factions, sure. I'm iffy about lycanthrope factions, because the idea of them even forming a pack seems completely ridiculous to me, let alone anything more complicated. What I
do think would work and have mentioned before, is to tie them to the cult of Hircine. Worshippers of Hircine are probably the people least likely to kill someone for being a werewolf, and may even honor them. Shrines/temples to him could be safe havens, as well as a source of quests and rewards for them that actually makes sense.