The major overhaul will not be to core mechanics or even to specific abilities, although there will still tweaks to those. Enochian was one in particular that was said to be receiving a way to make it less punishing to your dps numbers if you lose it mid-cycle.

The mayor overhaul will be to cross-class skills. Currently each cross-skill is specific to a class, and each job can only draw on two class' cross-skills. This leaves some key abilities missing for certain jobs. E.g. Keen Eye for ninjas. The change will be that jobs will instead pull from an archetype list of cross class skills instead of two classes cross class skills. This means that all melee dps will have invigorate, all dex dps will have keen eye, all casters will have swiftcast, etc regardless of their secondary and tertiary classes. In fact, tertiary class will no longer be a meaningful phrase since only the secondary class will still have impact for those jobs that require level 15 in it to unlock.