I've just been discussing this with a friend of mine. We both have experience in multiple systems. This is what we have devised.
At Rank 1 you choose a speicalist environment. At Rank 5 and every fifth level thereafter (10, 15, etc) you can choose a new environment. The environments are: Arctic, Cavern, Coastal, Forest, Freshwater Wetlands, Highlands, Jungle, Plains, Rural and Waste.