Right now we have 1 class going into 1 job. When they decide to include more job options for said class they could always include new abilities for classes and lock some when switching to jobs.
Let's take Gladiator as an example: right now the class is a pure tank with a lot of tanking abilities so it's natural that Paladin gets access to everything GLA has to offer. If they want to add DRK to GLA all they would need to do is add a new set of abilities to the base class (thus making classes even more varied than they are now) that aren't tank oriented and then ,when switching to DRK, you'd lose all those tanking abilities while keeping the damaging buffs and add more with job exclusive abilities. The opposite for Paladin.