Dawnguard, skill line, I realize the fighters guild has some that are akin, but adding this skill line would please and also benefit those whom just don't wish to play a werewolf or vamp.
Vampire Lord, skill line, only vampires can take this on, and would give an entirely separate skill line to players whom are infected with vampirisim. It would allow for the vampire lord form which would function similar to werewolves in attacks, and can only be activated at vampirisim 4.
Necromancy, some players would probably like to summon a skeleton or such for combat, or have additional spells to use in game. Adding necromantic spells and abilities could be a cool addition.
Pyromancy, mages at current have to use a staff to access the capability of fire, why not add a skill line for using fire spells, which would have an ultimate of conjuring a fire elemental.