I honestly can't see ANY classic FF job lining up with the current classes well enough to have classes able to equip more than one job. Maybe (MAYBE) GLA -> RDM and PUG -> THF, but even then, with the 15 skills the base class gets that's kind of a long shot.

So if they're going to keep implementing new classes to match their goal of having classic FF jobs, they may as well get rid of classes entirely, stick with the job names, and if they want to promote a "specialization" type of class structure like they do with jobs now, they can just give players to the option to "Limit Break" (term pending) their jobs after level 30, giving them access to only two other class's skills and boosting their stats, just like equipping a soul crystal does now.