Stick with GEO/RDM. RDM and BLM each have MDB III at 49 which will enhance your nukes potential by a significant amount. /RDM specifically will give you much more versatility in your spells with some enfeebles, some defensive spells, and cures. BLM and SCH would be lacking in this department.

The only reason to go /NIN would be at 67+ when you get to play with hexa-strike. The elem debuffs are not worth your time since /rdm will significantly improve your fast cast and damage.