Red Mage would need it's own class I think, or be one of those hybrid classes that were mentioned that would require both CNJ and THM, but I can't see that coming for a long while.

Blue Mage I think would work well as a tank coming from the Gladiator class. Just give them an ability at 30 to learn Blue Magic from monsters, there's plenty of unique defensive and offensive moves out there that mobs use.