It wouldn't be hard to make a geomancer job.

Summoners and scholars already have different versions of the same spell (summons). Just because a WHM uses all the CNJ spells as-is doesn't mean a GEO can't take a few and switch them to offensive or support. This sort of mental realignment would also play well with the quest to get the job.

Add some sort of interesting mechanic to the DPS rotation and you're off to a good start. Ground or environmental effects would seem appropriate.