I'll cover the demon hunter first since I've thought about it more. Lore-wise, they're in pretty large numbers from when Illidan was training up his troops, and we have two races, one for each faction, that could be one: Night elves (the original DH's) and Blood Elves. Now, after Illidan was slain, what happened to those DH's? It was never explained, but they could have remained in Outland, eking out an existence, but things have gotten so bad that they realize they need to align with their respective factions to survive. They could start at level 1 as new recruits (or reached the age that they can start to train, being born in Outland), or at a higher level, maybe lower than Death Knights. This part's the trickest to figure out honestly.
As for play style, DH's would be Warrior/Rogue with a touch of Warlock in the mix. They'd wear cloth armor, maybe leather at the most, because they prefer their agility, grace and skill to keep from being injured instead of bulky armor (and the runes they paint on their bodies could also have a damage-lessening effect, along with other effects, basically serving as the chest armor for a DH). Most of their damage would be physical, with the option through talents and abilities to augment their weapons with fire and/or shadow. They would have mana, but not to the extent other mana classes do, and most of their abilities would operate on cooldowns (off the global and on their own individual cooldowns), with only the abilites that use fire and shadow using mana. DH's main stats would be Agility, Strength, dodge, parry and hit, relying on high dodge and parry along with their runes to avoid and mitigate damage. For the talent trees: one would focus on their physical damage and skills; one would augment their fire and shadow; one would increase survivability and secondary stats. For weapons, their preferred would be the warglaives, which could be adjusted to be two-handed or dual wield by combining and separating (two-handed would be better for the more cautious and precise types, dual wield would be for the reckless and damaging types). Various abilities could be: powerful strike with both weapons causing heavy damage and bleeding; 20% of damage dealt is also dealt as fire or shadow, or 10% of both at the same time; panic skills to increase dodge and parry for a certain time; augments to the runic armor (more damage mitigation, some elemental resistance, increased mana regen).
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And now to the Bloodmage. Because of it's reputation and stigma, only a few races would practice it (for example: Humans, Worgen, and Dwarves on Alliance side; Blood Elves, Trolls, and Forsaken on Horde side). Though I just started thinking about it, it would be a neat class to play, since it revolves around the manipulation of blood, both yours and others, which opens a lot of possibilities.
The resource Bloodmages would use would be (of course) Blood, and it would work like this: it slowly regenerates, like Energy and Focus, but at a slower rate. However, you have the ability to extract the blood of enemies to refill your Blood, which also does damage and is one of the main attacks for the Bloodmage. In turn, you use this Blood to power your spells and abilities. Firing Bloodbolts, throwing Blood Bombs, draining the blood of one enemy and firing it at another at the same time, and manipulating the blood inside your foe to debilitating and damaging effect.
For example, you could raise the temperature of a target's blood, burning him from the inside out. Or you could chill the target's blood, slowing and freezing him to death. You could form bubbles in the blood to stun a foe, or make their blood burst out of his body in an AoE that hurts him and surrounding foes. And in emergencies you can use Blood to heal yourself a bit. You could also cause a slain foe's blood to spread from his body in a puddle that works against other foes, slowing or DoTing similar to a Hunter's traps. At high levels you could even make a Blood Golem, similar to the Necromancer's of Diablo II, with its own abilities and nuances with the Bloodmage.
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So, there's my ideas. What'cha think?
But some mechanics are already available, like how Blood could work, and how you can use your enemies' blood to attack, damaging them by first pulling it out and then attacking with it, or just pulling it out to replenish your Blood. A good AoE could be Blood Nova, where you make the foe's blood erupt from their body damaging them and everyone around them, it would do less damage than Living Bomb and the warlock one, but it would be instant and able to use again sooner.
