Well, if breathability is a prerequisite to colonization... hmmm, it gives the non-oxygen breathers one heck of an advantage, no?
Would it be possible instead (is it even moddable??) to decrease the facility ratio of domed planets even more instead?
On the plus side, all those 'useless' planets will make for many more races per sector with greater chance of peace, which would make for far more interaction/politics wouldn't it. On the down side again, who wants Vorlon neighbours?!
