I thought the opening scene showed an ARC getting buffed that added flame to the arrow..
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Battle Mage!!!!
point is add that sh*t!
It needs to be something that requires cooperation between party members, and something that offers enough of a reward (i.e. damage, enmity redirection) to be worth using.
For instance on Thaum, with any reasonably good gear/materia equipped, it is very easy to pull hate off tanks. You can either pace yourself (boring, but effective), or you could have another option that allows you help the tank do more damage and keep things under control at a damage/mp tradeoff.
EDIT: For the FFXI players, remember being 37ish and doing SATA+skillchain on the tank? The damage was good, and the tank had it much easier. The cost was cooperation between players.
I feel like the combo system in place actually lends itself really well to this idea.
Thinking along the lines of...
>Thaumaturge targets Gladiator with Thunder Spell.
>Lightning effect goes off around the Gladiator's sword - he now has a combo timer activated (similar to when doing a self combo).
>Gladiator then has that 10 second time period to perform a weapon skill (Specific? Any?) that will have the added damage and effect of thunder.
Simple and uses a system that already in place.