The visual style of Thavnair, and their research into the souls of things, (Notably our Zodiac weapons) would suggest That would be the most likely place for Blue Mage, though I wonder what other jobs would come from there. Really, Blue mage may be easier to implement than they might think. Instead of allowing you to set subclass actions, that could be removed in favor for equipping spells learned as a Blue Mage, with certain core spells or abilities learned via questing.