This is something I have tried to think how they could do it if you are talking in the sense of bring back more traditional classes.

How could they keep CON and then still have a WHM and BLM as another two separate classes, maybe make WHM cure's more powerful and BLM's nukes more powerful, but then still if you levelled both you would just put all the best abilities onto one of the classes and make a super mage.

If you just mean in a sense of how the game is with the current classes this as well troubles me. People moaned about being pigeon holed into levelling certain classes and things for sub jobs in FFXI. What are they going to do now when you have to level a lot of classes to cap just so that you have those select few abilities from them all to make your class better than average.