FFXI solved this by instead giving GEOs the ability to change terrain, verses the magic of the skill changing with the environment, using a pet-like tool similar to turrets. Essentially, they had multible versions of skills like Shadowflare and Sacred Soil. They could work as a viable support if the Devs go that direction.

FFExplorers expanded it further by letting GEOs lay traps that mobs could trigger.

Classic GEO had a much rougher appearance than the Feng Shui Warriors or Green Mages we see now. Particularly with FFT's vaguely Native inspired appearance. Another niche we could fill is the desire for a less heavily armored tank, using leather skins instead. This version of GEO used maces and warhammers,