The problem is white mages are evolved conjurers, and therefore already established as the druidic Disciples of Magic. In order for a geomancer to work, it would have to be close to conjury without being a healer. The geomancer in FINAL FANTASY XI and the arcanist that would have been implemented in FINAL FANTASY XIV 1.0 worked similarly as well.

For those who don't know, the geomancer in FINAL FANTASY XI would have elemental and enfeebling magic in addition to geomancy, which was typically cast with a handbell (the series' traditional weapon for geomancers) but didn't require it like a ninja required tools (ninjutsu was cast by consuming ninja tools from your inventory instead of MP). Geomancy would create aura bubbles around the caster, the caster's pet luopan or even another party member in order to buff allies and debuff enemies that the geomancer had enmity on. The geomancer also had a job trait called Cardinal Chant that gave bonuses to his/her magic spells according to his position relative to the enemy. The arcanist that would have been implemented in FINAL FANTASY XIV 1.0 would have used a mechanical cane to animate distaves, creating basically the same play style.

Anyway, as far as FINAL FANTASY XIV is now concerned, the geomancer would basically have to be another DPS job. It couldn't work as a tank. Think of it as bringing to bear all nature's wrath against your foe.