Whether GEO is a healer or not, I wouldn't be upset if unlocking GEO required getting CNJ to 30. Don't confuse this as a split job like ACN, but rather like ROG needing another class at level 15 to unlock. This way, the lore is there, not needing to be changed but applied in a different way - perhaps as a caster that gets to really make strong use of Wind/Earth/Water magic to be more of a counter to BLM.
For example, leveling CNJ to 30 unlocks that other door in the CNJ guild. The Kugane Geomancers open a school of geomancy in that room. GEO is then unlocked with its own story and lore but piggybacked on CNJs lore. No shared skills, no shared leveling, just two similar schools of magic working together. I can think of several jobs that could benefit from a system like this to see their inclusion in the game (such as ARC being required to unlock BRD but progressing into RNG for those that don't want to play music).