i don't think that the class/job tree will be expanded vertically but horizontally.
yoshida said that if he had to ignore the game balance he would implement the red mage but he couldn't.
the thief would be good as a gladiator job since it can already use daggers and the weapon defines your class in xiv; the same could be said of the ninja since katana means literally sword.
the samurai could be a job based on a great sword class.