Let's start with what would this class bring to the table: a playable warden, a playable demon hunter and a hunter with no pet all in one (separate specs). It would bring a regular type of tanking class, a new unique way to heal and dps at same time.
Background: Wardens... demon hunters... shadow rangers... all different sides of a coin. There's the prison wardens, the prisoners themselves, and the snipers in the prison towers, all having destines intertwined yet so different. One simple action can turn a warden into a prisoner, a prisoner into a sniper... such is the way it has worked for generations. Did all demon hunters start to study the arts on their own? No, they studied it from Illidan mostly... but then, why is there a demon hunter ghost in Darkshore? Because some studied it before he was set free. Some wardens were convinced by Illidan's dabble into fel arts and after studying him enough and taking notes in secret, they left with their newfound knowledge into self exile. Wardens turned convicts by simple actions. On the other hand, some wardens stood for so long in the dark that they instead learned how to manipulate it and the shadows. Not only Illidan's wardens did this, as there are many in the world that live in darkness.
After the Cataclysm, those that had layed hidden have returned to their people. While some aspects of their trade were not accepted so easily by some races, the fact that the teachers preached these ideas "just so you could defend better against them" was accepted as good enough motive by the leaders for the shadowy figures to teach recruits some new tricks.
Class colour: light grey
Resource: bar similar to moonkin sun/moon thing, but with a few twist which I'll detail. The extremes are called Hatred and Order though.
Races to which it is available:human, dwarf, gnome, night elf, draenei, worgen, orc, troll, forsaken, blood elf, goblin.
Why these races? Because either they live in underground or shadow or manipulate it (human, gnome, dwarf, goblin, night elf, worgen, troll), they have magical wardens in their ranks (human, night elf, worgen, orc, blood elf, troll) or like to explore the forbidden arts of fel magic to a certain extent (human, dwarf, night elf, worgen, orc, forsaken, blood elf).
Detailing of resource system:
0-100 type resource
0-50 is Vengeance, 50-100 is Order.
You have abilities that increase Vengeance, and abilities that increase Order. When you reach 0, you can no longer use Vengeance abilities, you have to increase your counter towards Order (Order abilities don't get bonuses or something like moonkin does). When you reach 100, you can't use Order abilities anymore, you need to decrease your counter toward vengeance. When your counter is between 40-60, you get bonus to damage and defenses. If you're lower then 40 or higher then 60, the bonus is lost. There are a few exceptions, like Permanent Immolate from Demon Hunter spec which, if activated while in the 40-60 radius, will retain the damage bonus until it is deactivated.
1. Warden - tank spec - this is a sort of dodge tank, the idea is to get hit as little as possible and your abilities reflect it. Many of your abilities that lower physical damage done to you move your bar towards Order, while those that lower magical damage done to you move your bar towards Vengeance.
A selection of possible abilities:
-Cloak spin - you spin your cloak around you creating a partial confusing effect on enemies who don't know where to strike, lowers chance of being hit by 5%, can stack up to 10
-Spirit of Vengeance - creates a Shadow that looks exactly like you, with exactly the same life, buffs, debuffs etc that does half your damage. It sits with you for 30 sec. 5 min Cooldown. If it lives at the end of the 30 seconds your life is doubled for 10 seconds.
-Vengeance Blink - take the place of your Spirit of Vengeance (can be used to change spot or to increase your life again)
-Hooked chain - you spin a chain with a hook on it around you, making your enemies more likely to attack you and knocking them down.
-Hidden daggers - you throw daggers in the air that target 5 enemies. If they don't move in 2 sec, they get hit.
2. Demon Hunter - dps spec - basically the demon hunter works so that the more damaged he is, the more damage he does. Also the idea of blademaster could be fit in here.
A selection of possible abilities:
-2% more damage against demons - passive, not much to say
-Flame of Azzinoth - a fel fireball falls on the enemy, if it kills them it summons a Flame of Azzinoth minion that follows you for 15 seconds, attack stuff and reveals targets in stealth.
-Permanent Immolate - ignites you dealing damage to all the enemies around you until your Vengeance bar reaches max, at which point it stops.
-Mana Ignite - using your demon hunter abilities, you ignite the mana of an enemy, setting them ablaze and lowering it.
-Dual strike - use your weapon to hit an enemy and stun them for 2 seconds, then hit them for 5% health damage.
3. Shadow Ranger - dps and healing spec - as a dps you are a ranged only physical damage dealer using shadows to outmanuever the enemy, as a healer you use shadows to protect your allies (think similar to disc priest, you put shields to prevent the damage rather then heal through it all). Also priestesses of Elune could be fit into this as they're ranged healers.
All your healing abilities increase your bar towards Order. Some of your dps abilities increase bar towards order, some lower it. And here's the catch. To heal you must also deal dps to decrease your bar so that you can cast healing abilities, otherwise when you reach 100 you can't heal anymore.
-Shadow Steal - Steal parts of the shadow of the enemy, dealing X damage (this will be an ability to move the bar towards Vengeance)
-Shadow Pact - bond a shadow with a selected mortal (this is a shield, for healing), the shadow protects the creature and stiches its wounds.
-Shadow Arrow - shoot an arrow that creates a shadow inside the enemy, dealing damage for 15 seconds and revealing their position.
-Nightmare Gaze - look at the enemy and have a creepy shadow scare them while you make a hasty retreat (this fears the enemy on the spot for 2 sec).
-Dark guise - shadows surround you, as long as you don't move, you are invisible (not in stealth, invisible like mages, but only on the spot!)
-Plague bullet - shoots a projectile filled with plague that explodes in the face of the enemy dealing some damage and some damage per second afterwards as a poison.
-Afterglow - using the light of stars to apply a debuff on enemies that lowers their damage and makes them glitter in the dark, thus making it impossible for them to stealth or go invisible for 15 seconds.