They already exist. And people that have levelled up DPS and ignored playing in the tank/healer roles will have at least grasped the core of their jobs just through observation if they've levelled even halfway sufficiently.

Considering this new format of being able to level these jobs from 1, perhaps they could meet in the middle and make some of this group stuff more mandatory to their class quests. A dark knight may sound like a lone wolf, but maybe for lore purposes, their mentor figure could push them to embrace the assistace of other jobs. Just an example. One that may pave a way for character development at that.