Implementing their classical Terrain ability as bubbles would probably be fine, in particular if they were Spells so they could be relocated at will. And with SMN now being about one-off big summons, the role of DoT mage is open for GEO to step in. I imagine, for example, a core similar to the following:
- An instant Terrain bubble Spell that is itself a DoT, and can be relocated at the cost of a GCD.
- A targeted DoT Spell that is only usable on enemies standing in the Terrain, or that is usable any time but only does worthwhile damage on enemies inside the Terrain.
- A single target direct damage Spell that does medium damage to enemies. If the target is inside the Terrain, the Terrain performs an extra attack on them.
- An AoE direct damage Spell that does minor damage to enemies. If any of the targets are inside the Terrain, the Terrain performs an extra attack on them.
- An Ability that consumes Terrain, and does instant damage to everything inside based on the remaining duration.
As for the elements, we haven't had any post-meteor jobs feature water magic. The lore explicitly excludes water magic from white mages so that this exclusion can be cited as the impetus for the Flood Calamity that ended the War of the Magi, so GEO wouldn't be stepping on anyone's toes by getting spells like Water, Drown, and Flood. And neither CNJ nor WHM got Quake and Tornado, so those spells are also up for grabs.