you could have something along the lines with a vampire faction a werewolf faction (i so want them to put werewolves in) and these factions could be at each others throats all the time where as the mage faction will be at the necromancers faction it would just bring more quests into the game that i think would be pretty cool e.g. in oblivion if you were a vampire, other vampires would still attack you even though your the same and if you were some outlaw the bandits would still want your guts on a stick. so if you introduced factions in the game it could lead onto game changing decisions like quests where lets say your a necromancer you will be against mages at the mage guild and your mission as a necromancer could be to destroy the mage guild same as if you were in a bandit clan you could attack small villages and take small parts of land.
with these set factions you could go deeper e.g. if you become a werewolf you could get 3 separate parts in their faction e.g. one part of the werewolves take is as a curse and the people just want to live ordinary lives but they are cursed so they will chain them selves up to prevent anyone from being harmed or the will go out into the forest away from anyone when they change. the other type could have a major situation with the vampires and their only objective is to exterminate the vampires (and vice versa which could make enemies) these types could control their from and not take much interest in humans. the other type could just be purely wild where it is just pure blood thirsty beast and will attack anything.
obviously you wouldn't want factions of mud crabs and stuff so it would be reasonable but overall i think it would make skyrim really enjoying to play cause it opens so many opportunities for the player to experience. what are your views?