i think a good 7th role would be a time mage considering the content in the games.which would be a time based mix of support and debuff.
they would get haste and slow, and the dont move and dont act actions. Their Atb would automatically refill 25% faster as their special class ability
They would also get gravity, which would work like the sentinels ability to extend the duration of the stagger gauge, only it would be a status effect, and it would stick even after the time mage has been shifted out. since this is what makes the time mage vital, gravity will work on everything. probably would take up the whole atb to cast or something.