Class morphing would be a good idea.. so your not "adding" a class per say but just expanding current class's.. this also can open up more class morphs later on..
Templar could morph in to a Justicar focusing more on damage and cc
Sorc could morph in to a geo mage or necromancer
Night blade to a bard a more support/healing morph or a ranger/natural pet class summoning
Dragonknight could morph into different aspects of dragons such as green dragon for healing or black dragon for range direct damage/ debuffing
Things like that would be cool to have and give more options... also you could do as a substitute for vampire and WW so you can morph one of your main skills or be a vampire/WW not both.. that would give a lot more options for variety of endgame builds.
Warrior Monk, like others have said, but with a basis in already established lore - Khajiit have three martial arts styles called Goutfang, Rawl'ith Khaj, and Whispering Claw, which could be the names of the three sub-skills. You can already read about the various temples of the Two Moons Dance, where it's trained. I think it would fit in well.
And yes the primary reason I'm suggesting it is because it's Khajiit-related.
1.Bloody Orb ( fires a red orb which explodes on contact, it can be slow like the Harvester orbs )
2. Blood Pool ( a pool of blood emerges under the enemy causing slow )
Morph 1: Blood Boil - Does a ton of DoTs Morph 2: Dried Blood effect now snares enemy
3. Sanguines Blessing - Buff
4.Raining blood - AoE effect, similar to bow skill Barrage
Morph: Infected blood - causes disease damage Poisoned blood - causes poison damage
5.Blood Atronach - summons a blood atronach to fight for you. ( should be similar size to Flesh Atronach)
Morph1: Boiling Atronach - Atronach does increased AoE DoTs, when it dies it explodes into a pool of blood.
Morph2: Instead of 1 large Blood atronach ( similar size to Flesh Atronach), summons 3 smaller Blood atronachs ( similar size to flame atronachs).
6. Bloody Judgement - Blood flows from the earth into the character and the character then shoots it out into all directions spraying everything around it with blood. Ult does DoT
Morph1: Not sure yet
Morph2: Up for suggestions
Sacrificial Magic
Can either be used as a supporting skill line to sacrifice necromancy skills for unholy wards / buffs.
Daedric skills ect. ( Not the lame Daedric summoning ) I am talking about Oblivion skills
For example summon oblivion portal and 1 of 3 different types of monsters enters the plane and it's random which one.
I just quickly came up with all of these right now. But I have thought about blood magic & Necromancy before so I had some ideas but never in this detail.
I also second the druid suggestion mentioned above