There are currently three classes that would clearly benefit from giving them two distinct roles:
  • Arcanist, where SMN and SCH are each role, and both would get virtually no changes.
  • Marauder, who could be given a DPS version of WAR with a more complicated & interactive rotation, while keeping the current WAR Offtank.
  • Astrologian, who would get the Nocturnal & Diurnal Sect specialization that is desperately needed to give the former buffs.

There are six more that you can argue for as well, but I'll admit they're all a stretch and go against the design SE has set out so far:
  • Bard and Machinist can trade their TP/MP restore utility away for a pure DPS role (Probably taking WM/GB with them).
  • Black Mage could flip Astral Fire/Umbral Ice around and have their rotation tweaked into an Ice specialization, ideally restoring the proc gameplay Enochian took away.
  • Dragoon could potentially tank cause they've got a solid enough base kit and the armor to do it with more support.
  • Dark Knight can follow the same logic as Warrior to get a DPS role, though it would require more effort.
  • White Mage would most likely get another healing role, probably focusing on big heals and procs instead of regens, since Conjurer supports that concept already.

The question from there is: What do the rest of the classes do? Paladin isn't suited to DPS, so the only real alternatives are a tank with more AoE and offtank support, some sort of utility DPS spec like Bard/Machinist but with Auras, or a hacked together healing role, in order of feasibility.

Also Monk and Ninja are very self-contained classes that are difficult to modify meaningfully, so my only guess is that they would get new kits that experiment with melee roles that don't rely on 1-2-3-1-2-3-kill-me-please, since the combo system feels overused now and would feel more oppressive if multiple roles were given to each class and at least a few didn't try to branch away from it. However, it's not clear what that would look like, at all. My guess is Ninja would get more DoTs and cooldowns that gave positional bonuses while Monk would stack and spend GL rapidly on various ogcds.