I'm curious exactly what worlds are covered under "airless planets"
Rocky planets with no atmosphere, obviously, but how about icy rock planets? Ice planets? High metal worlds? Does active volcanism count as having an atmosphere? I would love to drive my buggy up to a silica geyser...
Also loving this approach - first airless worlds, then worlds with atmo (possibly gas giants) to get that right. After that, oceans and ice flows. Then flora and fauna. It's a logical progression that gets us playing sooner, while still building the complexity up from least to most.