So I started to think about how Skyrim will be using perks for character development and had a sudden thought: why not develop the gameplay of vampires and werewolves through the use of a special type of perks?
Let's say that, when you become a vampire or a were-creature, you only have the very basic traits connected to it: as a vampire, sun harms you and you need to drink blood but you gain some relatively small advantage; as a werewolf, you need to kill every once in a while and are vulnerable to silver, and you gain some other kind of advantage.
Now, the game keeps track of how much blood you drink/how many victims you kill, and use this data to mark your "progress" as a vampire or werewolf.
So, the more you act as a vampire or werewolf, the more powers and limitations connected to your condition you will gain.
For example, a vampire could gain the perk of masking his/her undead condition if well fed; could gain a powerful/cheap Absorb Health spell but lose the ability to heal with Restoration magic; could gain a power that gives increased combat abilities at the price of becoming bestial-like (claws, fangs, fiery eyes and the like) when the power is activated; etc.
Of course, this progress as vampire/werewolf is in addition to the normal character progress (levelling, rising skills, etc).
It's just a skeleton of an idea, just to give you the "concept".
The point is: the more you play as a vampire/werewolf, actually acting like one, the more you will gain traits that characterize your pc as one of these creatures.
Also, this would give you a lot more control over the way you want to play your condition: you could go for a sneaky type, a brutal warrior, a master of the dark magics that your new condition allows to unlock, etc.