An old school Geomancer would be awesome, but the implementation seems quite difficult. For example, in FFT maps are quite varied and each one has several different tile types. In an MMO areas are much larger and typically uniform in their scenery. It would get quite boring if you can only cast Ivy for 30 levels unless you fight on that beach over there or that pile of rubble in the corner of the map. Weather is also something that is infrequent and wouldn't be viable as a source of spell variety. I suppose if you learned spells through these processes and were able to use them elsewhere it might be possible...Perhaps storing certain spells for later use?
Not difficult. A simple solution is to make the terrains unlock different spell lists. e.g., a variety of plant magic becomes available while on fertile soil. There are other ways, too, but this one is simple to implement.