Something me and my fiancee have discussed is the Geomancer. The geomancer is utter crap in the other FFs it was in. However, It could actually really really work in a game like FF XI and not throw the balance too much.

Part of the biggest let down in other FFs with Geos is that they could not choose their attacks directly. The attacks were fairly random based on terrain and weak making them kinda suck. Mostly the menu systems just never allowed for a really intricate use of Geos.

It would be similar to the Bard but unlike the Bard it would actually use attacks without using MP (I know the Bard can but... come on... really).
This might easily tip the balance you would think being able to basically nuke without MP but I'm not so sure.

Mog was a Geomancer/Dancer in FF VI. If you take the dancing part out you could have the same pros and cons on a Geo for FF XI. A Geo could use only the terrain around them to their extreme benefit. However, if they try to use other terrain abilities the ability would likely fail, not be usable, or be severely weakened.

The balance actually works for a lot of mobs. Think about the worms that are so often in rocky areas but not weak against stone. This would put the geomancer at a loss... therefore not making it over balanced.

Additionally, the visuals for a potential Geomancer would look very cool I would think.

So...
-Light nukes without MP
-No major balance issues
-Potentially cool animations
-etc...