Sub jobs are a nice idea in concept but they are very hard to implement properly in an MMO, especially when the balancing is already quite fickle.

I too would like to see more character growth, but at the end of the day its not that kind of game and ultimately would require too much rework, it would also be nigh impossible to balance, even in FFXI, the term "balance" is laughable, people already complain about the attitude towards the meta comp, imagine that but also taking into account a sub job.