Melee Healer: Sorta tricky to implement, but possible
Magic Tank: Not DRK, but a Tank that exclusively has spells
Counter Tank: already mentioned before. But the foundation is already there with WAR's Vengance and the mob exclusive Reflect and Directional Parry

Now for the Nitty-Gritty of DPS. This is simple to add, they find a concept and base the Job around it. Like a Debuffing class with plenty of CC abilities (And high DPS to compensate for the fact that CC ability are useless), Or like the aforementioned Geomancer idea (Again, groundwork for that has been done since Wanderer's Palace HM and Ley Lines/Sacred Soil/Asylum/Collective Unconscious/Sacred Prism).