I just thought of this and I'd like to discuss.
Maybe all this "No base class involved" work in a different way (I'd like to):
You will do your DKN, AST and MCH (my guess on shorts) quest with any class you want, right? What if you earn their crystals and you're free to use them with whatever class you want, no mandatory base class? We already have a role change depending on the crystal you use (ACN -> SMN-DPS/SCH-Healer).

People could have a CNJ base class and use AST crystal, they would remain as a Healer.
People could have a CNJ base class and use DKN crystal, they would be a tank capable of using Medica II!! But with low HP and only cross class CDs.
People could have a DPS base class and use DKN crystal to be excellent offtanks.

Of course some combinations will be better than others, but we will have a lot more diversity and could do new and wonderful things with these "hybrid" classes/jobs. A lot of dumbness could occur too, for sure, but people will learn the best ways.