What about a class that adds temporary buffs to the group as a whole as well as individual players that only last for a short duration (5 - 30 second up-time per specific abilities. Large cool downs possibly last 1-2 minutes) and modify damage done, taken, healing done, taken, instill temporary abilities to use while certain buffs are active. Basically you add interesting active buffs with abilities you can only use while the buff is active on you be it for the full duration or specific amount of charges / uses / stacks, or apply debuffs to a target akin to most other class debuffs. After writing this, it sounds like a hybrid really, but it would be one of your more interesting ones.