As a whole, I think terraforming could use more detailing for what happens once temperature and atmosphere concerns are addressed. For example, atmosphere without an ecosystem could degrade with oxygen turning to CO2. Pollution could also be an issue worth adding, requiring you to use terraforming installations and workers to remove dangerous gases from your otherwise breathable atmosphere.
I would like to see terraforming fleshed out a bit, even if it's just fluff that goes on automatically in the background. For example, an indicator of the state of a planet's biosphere. You can give a Mars-like planet an Earth-like atmosphere, but you're still going to have a planet of nothing but dead regolith and barren rock unless you gradually introduce life to it. Building an ecosystem from scratch would have to start with basic pioneer plants like algae, mosses, and lichens to build up soil for more complex plants to grow in, which then allows for animals.
To keep a biosphere simulation fairly abstract, just break it down into a few categories:
Lifeless -
The system body has never had life. If left as is, it probably never will. Microbial -
Only single-celled lifeforms exist, either because conditions are too hostile for more complex forms, or there hasn't been enough time for it to develop further. Simple -
A limited biosphere of simple pioneer plants and small hardy animals exists. Complex -
A diverse, fully developed self-sustaining biosphere exists. And some special cases for when things go wrong:
Degraded -
The system body's biosphere has been significantly disrupted, perhaps from a natural disaster, minor xenoforming, or bombardment. The biosphere may adapt and recover with time. Dying -
The system body's surface environment can no longer sustain its former biosphere, either due to stellar evolution shifting the habitable zone away, or because significant xenoforming has occurred. Extinct -
The system body once had life, but it has been extinguished. On a lifeless world, once terraforming reduces the colony cost to, say, below 1, have the biosphere category change to Simple as organisms genetically adapted to the colonists' preferred environment are introduced. After enough time at zero colony cost, further increase the biosphere category to Complex, simulating the colonists adding more advanced lifeforms to bring about ecological succession.
Handling alien ecosystems would be. . . somewhat less simple. One species' ideal environment is another's horrible death. We can see just from the examples our own Earth provides that life can take hold almost anywhere with an energy source and a fluid medium. Perhaps the planet's biosphere could have tolerances and ideal conditions in the same way sentient races do, and if the environment is changed too much, the native life dies off, going through Degraded > Dying > Extinct as you terraform the planet away from its original conditions. It would make for some delightful ethical dilemmas such as "that vital strategic planet with the methane/ammonia atmosphere is full of life, would you go ahead with terraforming and kill everything on it?" Conversely, finding a rare natural zero-colony-cost world would be all the more special if it also had a Complex biosphere on it, a true garden world.
You might also find Degraded, Dying, or Extinct worlds naturally too, with Dying/Extinct particularly around B/A/G-class stars nearing the end of their time on the main sequence, and Extinct worlds around red giants and white dwarfs.
