I think it would be systematic and fair (and add a lot of variety) if each class had an offshoot job based on each of the three roles, Healer, DPS, and Tank.

They're on the right track with ACN's jobs not being the same role twice. Add a Tank role based on ACN (maybe Time Knight, for the OP's amusement) and we have one class that can completely and fairly split into each role depending on the player's preference. Time Knight based off ACN would need a pet to go with it, however.....