I've always wanted a reversal of fortune type buff effect... Where instead of taking x damage (a cap to this), you would be healed, thus negating the damage. Or an defensive-offensive spell that prevented x damage but reflected a portion of it back to the enemy.

So for example... It would shield 100 points of damage. If a mob hits for 105, 5 damage goes through the shield, and 100 damage is reflected onto the mob.

A Stop ability would be fun too. Something that stops a skill mid-cast for a second or so... I'm just brainstorming here, so there's no logic whatsoever behind this. :P