I only have a concept, not a full work up for the job. I have no experience with Green mage in any other FF game, but was listening to a youtuber suggestion and the idea of "Green Mage" or "Druid" interested me.

Basic Weapon would be Staff. Your basic single vs aoe rotations.

Job Style would be what RedMage should have been a buff and debuff job without the "Support Jobs" mentality. Basically, AOE extra elemental damage for attacks, and Elemental Spikes for Shield/damage over time. Maybe, a summon "pet" job. Think Golem or bear?

Thematically, magics would still be single vs AOE, but visually spells would be based around plants/tree/earth,water, etc.

For example;
Bramble, AOE Damage over time that poison while mob inside.
Vine, Single target Damage with "Stop" effect.
Stoneskin, earth shield with spike damage.
Tornado, Single target Wind damage.
Mud, Slow spell
Spore "Sleep"
Quake AoE earth damage.
Malestrom AOE Water Damage damage over time "drown".
Blink - Self cast avoid one attack
Etc.

The Lost City of Amadopor, could be a cursed site from a mad druid/green mage