[Fyron]

Let's say you wanted to expand the stock set of breathable atmospheres from 5 to 10. What would you add? All such types have to be feasible for energy production/storage use by large organisms. Noble gases like Argon are right out. None is an exception, naturally.

The stock list is;

None, O2, Methane, Hydrogen, CO2

My current list of candidates is:

Ammonia, SO2, Chlorine

Consideration of Nitrogen transformed into a more suitable (in my eyes) Ammonia. Fluorine was considered, but it is possibly too reactive. More importantly, it would end up with the same exact color planet as Chlorine (pale yellow-brown), which would make it visually indistinct in-game.