One reference needed: Vincent from FF7

He could transform with a limit break.
Lets say you could have a job that could do such things, I believe for in order for it to work, it must be an attack, and not a form. not like WoW's shaman and druid.

Perhaps initiate the attack and you briefly see your char turn into a monster to attack the enemy, then your character reverts back to normal form after the attack is completed. Worked with summons in earlier ffs so why cant it work now?

Details would be left up to Devs