I don't think I quite understand the impact this would have.
I mean - there's already genetic engineering, but you're talking about the impacts of terraforming on wildlife?
The current terraforming system is complicated in a sense (building air pressure, percentages of oxygen, temperature concerns, filtering out toxic gasses, a nod to melting ice caps and other caps related to gravity, etc.), but reducing the actual task of terraforming to just adding and subtracting gasses has always seemed a little simplistic to me.
I think about things like the Red Mars trilogy: introduction of plant life as a means to terraforming (just another way to swap those gasses, really), orbital mirrors, tapping into geothermal deposits, etc, etc. Terraforming could be so much more interesting! Though figuring out exactly what that would look like outside of Sol... well, it's a little hard for me to imagine implementing it all. But exciting!