I've always seen this as a minor issue. What if I wanted to build the City of a Thousand Slums? Or if I wanted to be a jerk to my neighbors and ice over whatever valuable resource that I couldn't afford to take at the moment.
To implement any sort terraforming is not incredibly difficult. You'll just need to add entries to spell infos, than copy and modify the relevant entries in the spell trigger python file. I would perhaps disable terraforming for the AI, because I don't think they would use it very well.
A common sense approach would be allow individual terraforming with whatever level II spell type. Ice II could allow snow and tundra.
Additionally, a spell III could be used to cycle random bonuses... Chaos III to randomly swap a pasture resource for any other pasture resource.
