My money is on jobs being treated as sub-roles. Like how two tanks will tactically separate into Main Tank and Off Tank. Or how Scholars and White Mages synergize well in that one is reactive and the other Proactive.

I think the Arcanist is going to prove to be an exception to the rule, with a sister class having a DPS job and an Off Tank job. (I bet Monk for that.)

In fact, ill call it like this.
Gladiator = Main Tank/Off Tank
Marauder = Off Tank/ Main Tank
Pugilist = DPS/OffTank
Lancer = DPS/Debuff
Archer = Buffs/DPS
Arcanist = DPS/Proactive heals
Conjurer = Reactive heals/ Proactive heals

Of course, I could always be completely wrong. That's half the fun of this sort of mental exercise.