I love the idea there, but would expand on it and go a little different direction.
First -
Buff the Werewolves. Right now they are incredibly weak specially the further in the game you go.
Gear the buffs mostly toward Warrior.
Make it so thier buff is both in darkness and day.
Tier the werewolf disease like vampirism.
Extremely weak to Silver Weapons
Second -
Create a new disease. Wisp/Treeant/Something that screams caster
Gear the buff toward Casters
Make the buffs work primarily during the daytime
Tiered disease again.
Extremely weak toward melee in general....not quite right but only place my brain went
Create a base in one of the green areas of the world.
Third -
Rework Vampirism toward rogue a bit more (should be minor)
Leave them night only. Current system i think is good.
Give them a legit base to work from, like morrow wind.
Extremely weak arrows (wood)
Fourth -
Epic Quest Line.
3 Faction dispute, with alliances and backstabbing to get X faction to the top.
And make them exclusive...only 1 faction can be at the top.
Circle of lunch meat --- Wolves -> Vampires -> Wisp -> Wolves
or
These groups are splintered at first and come together at the end of a epic quest line to defeat a nest of dragons? mmmm
mmmmm suddenly i feel like doing a mod.
Good idea but I would rather do each part as a separate esp so people could mix and match with other mods. The point of only having the original vampirism disease return and be tweaked a bit for mages is to change the vanilla game as little as possible in order to maintain absolute compatibility with other mods.
For instance, I had a different idea for werewolves. Have sanies lupinus return again and have very rare random werewolves spawn with it. These werewolves should be able to infect you. The disease should function like the one in morrowind, where you turn into a werewolf at night with no control over the transformation. Spoilers related to lycanthropy in skyrim ahead.
Spoiler If you have the ring of hircine, then you gain control over the transformation but must still transform once every full moon. The point here being that the companions teach you how to control lycanthropy and without their teachings, you have no control over the transformation or the beast itself. However, if you obtain the ring of hircine, you do gain some ability to tame the beast, but not entirely. If you want to entirely tame it and never transform outside of your will, you will have to join the companions do their quests until you get to that point. This form of lycanthropy should also exist outside of the companions in order to maintain absolute compatibility. Unfortunately, this means that the only way to cure yourself is by doing the companions quest line and once you cure yourself, it should be impossible to obtain the disease again, like it is in the regular game.