Mobility wise, as long as they get a good oGCD tool I don't see that being a problem.[In my last concept I gave them Spectral Upheaval, that allowed the GEO to warp to and from enemies or party member(s)] Another alternative, could be that within their own Terrain Manipulation (ground effect), their spells could have a higher Spell Speed?
There are a lot of avenues. I'm excited to see what SE's Development team gives us for a GEO. Doubtful they'd translate our long-drawn-out concepts we do here, but as long as it gets some serious love, I'm sure it'll surprise us!