Every idea will find its fans in any case. How about a colony built of toilet paper? Or even better - who needs a wooden planet? How about a planet-sized space tree? And let's call it Yggdrasil! Perhaps the engineers are just space elves who live on this tree. The Black Goo - the sap of the tree. Aliens are just ants. It is possible that the tachyon vibrations of the tree can grow alien eggs in random places. That would perfectly explain the egg on Sulaco. Because we have already gone off topic.
P.S. On serious note - how about a colony where people live in hobbit-holes like hobbits?