They could even do a tease with them in the dark robes of a BLM (or if they add one, a new darkness-based caster), flashing Azem's Convocation sigil, just for a bait and switch.
What's wrong with buff mages?
Besides, the character being
means they're probably possess the aetheric reserves to make for a potent mage - maybe not necessary for BLM but every little helps. Rigid adherence to "what's been done" for its own sake can grow dull, and if they don't know how to make a mage play in an exciting way in a trailer, they just need to look at how other games manage to do it.