It's not so much a rework of the entire battle system as it is just a rework of the actions and skills available. They've mentioned that they're looking at changing the effects of or removing certain actions that are underutilized such as Featherfoot, Haymaker, Keen Flurry, and a few others. Some actions that do see use in normal rotations that should have useful effects that aren't exceptionally useful due to either fight design or other actions with the same effect might see changes (Dragon Kick, Rage of Halone). They've also mentioned it's possible that they might be doing away with persistent damage resistance down effects like Disembowel.
The only thing that's known for certain right now is that they're going to be changing the way cross class actions work. Instead of having to level alternate classes to use actions like Blood for Blood, they're going to be assigned to Roles and will be useable by Job's that fall within that pool. IE, Melee DPS won't need to level Monk/Dragoon to have access to Blood For Blood, Internal Release and Invigorate, they'll be able to take them immediately.