You could also add skills to each class that removes the "role" of classes, and have jobs add some skills and then allow those jobs to choose from the "focused" list of skills. For example

Lancer is a DD class and Dragoon is a DD Job. If you added skills for Lancer that gave it a tanking role but barred Dragoon from using Tanking skills you would build not only a use for Lancer itself (you could tank or DD with it) But also preserve the DRagoon job focus (DD specializeation)

If this was done with every class GLD, MRD, CNJ (Heals/DD) etc then not only would current jobs be a specialization/improvement on a specific focus. But also open up the basis for additional jobs that occupy a different role.

Maybe a a new job from Lancer could be a long staff using tanking job. And by equipping a longstaff this class can choose from a defined list of lancer abilities mostly consisting of the tanking type plus the skills that are rewarded by being this longstaff user.

And as far as a Dark Knight goes. By adding DD and tanking abilities to base classes you can now add greatsword as DK only Job weapons and say you choose GLD as the "Base" class you now have a choice of mostly DD skills for the DK while PLD gets choice mostly from the Tanking GLD skills.

ANd classes servive because though not specialized they can DD and tank. Switch as needed but not do as well as the jobs.

You get specialization with Jobs and a focused choice of class carry-over skills. And job specific skills.

You get versatility of Role and skill selection with classes but not quite the focused power from jobs. But you can Tank of DD with your GLD, or Lancer, or MRD.