The concept of advanced classes in an interesting one, but they already have plans for further customization at lv 99. It was announced for VanaFest 2010.

An increase in maximum upgrades is being considered for existing merit point skills and attributes. Preparations are also under way for the introduction of a new type of merit point for players who have reached the new level 99 summit.
http://www.playonline.com/ff11us/event/vanafes2010/

I would love to see advanced forms of the current jobs we have now, but I don't like the idea of locking into a certain form permanently.