Monks would be the only new class they could add that wouldn't be derivative of any other class or profession.
They could fulfill all 3 roles and be based in the martial arts.
Armor: Cloth, Leather
Weapons: All one handed and staffs
Ranged: Thrown
Healing Spec:
The first melee range non-casting healing class. The use of chants (for brief bubbles to friendlies) and quick movement could get the healer from one target to another. Think of it inspired by pressure point therapy and acupuncture. Could specialize in tank healing and really up the challenge level for raid healing.
Melee DPS Spec:
More focus on punch and kick specials. Have weapons for white and small amount of specials. Have a talent for specialization in fist weapons or staffs. Lots of very short buffs and debuffs through chants.
Tanking Spec:
Let the focus be on dodges, blocks, counters and mental fortifications. Think "evasion tanking" for lack of a better term. Dual wield tanking with the off hand acting as a weaker shield and/or weapon.
Primary Resource: Focus
Secondary Resource: Chi