I'm probably gonna get flamed as the ideas are kinda lame and simple, the ideas are just bare bones.

Tanks:
--Maybe Make a mechanic that a tank has to drag a monster / boss into a puddle of bad in order to give it a debuff to make a one shot kill move survivable debuff given like a virus or something that a DPS or healer may not want but a tank could get away with.

-- Dragging a boss to make it tether to an object in order to give it stacks of some kind. Also said object that tethers move about the room, making it so that the tank has to move with the object hence moving the boss.


Healers:
--Make healers cure something else other then party members to full HP in order to activate a certain mechanic. Like Having a healer Cure something to full from 0 in order to transfer mana (charge) a shield in order to protect the party from a one shot kill move.