It doesn't seem like alien villagers or slimelings require oxygen. I think it's interesting. Perhaps an interesting new mechanic to the game would be that these guys are poisoned by it. Sure the alien villagers wouldn't be in many situations where they would be in an oxygen environment, but I think they should have some sort of weakness, being immune to zombie attacks and all. Also a player would have to be more careful with their slimelings as taking them into a sealed space would be the death of them. It adds a new dynamic that players would have to plan for.