Not a super thought out idea yet. Thinking a Cave based rails shooter, generally pretty dark, lighting from the environment mostly I would think.
Maybe multiple biomes? Start above ground, enter the surface level caves, gradually turn redder/more sinister.
Will probably end up being more PvE than against a alien race or anything like that, maybe some cave monsters? Maybe a big baddie destroying everything within the planet? Ill keep thinking and developing this as I move through this section.