I had an idea for a new "Quarter job" unlock-able quest that would basically be 1/2 of your subjobs level. So for example: PLD99/DNC49/WAR24.
I don't think this would be overpowered for any one job (quarter job) because if you look, it's only going to provide a handful of procs, and many of the more meaningful abilities and traits won't come until 25 or later (such as WAR's double attack).
It might be a good quest to tuck into the new expansion, because it would give just a bit more power to 99's without being overkill.
There are many more priorities than implementing this idea (i.e. fixing the broken enmity system) but I just wanted to throw it out for some creative thinking