@StarshipAmelia tbh the first thing that pops in my head is something like the setting of Valerian and the City of a Thousand Planets – a vast multitude of alien races have managed to build a place where they can all coexist. so "crowded sky" references the fact that it turns out there's a whole lot of life out there in the stars. and also literally, this city/planet/whatever has a busy sky w/ all the various trade operations