Like how we have to level up two classes to unlock a job. What if we leveled two jobs to unlock an advanced job. Examples could be like Blackmage+Astrologian=Galaxymage or Time mage, or something along those lines. It could function like the crafter specialization where you can only have 3 advanced jobs unlocked. These new jobs can bring unique skills and traits that would otherwise not be available to the original job.
Instead of just upping the level cap with new skills, give people that enjoy maining one or three jobs the options to specialize them. You could make some pretty wicked combo jobs.