I did like the Alpha Centauri way of doing things, if you ever played that. Aside from the main/simple improvements (farm, mine, solar collector; every non-hill tile could be farmed AND have a mine or solar collector), there were "bigger" improvements as well. A Condensor altered rainfall patterns, basically making land around it more fertile (comparable to plains->grassland, I suppose); an Echelon Mirror made every solar collector on a tile adjacent to it have +1 "energy" (AC's equivalent of commerce) and some sort of supermine gave very high Minerals (production) and Energy (commerce) but no food.