Re: Interesting Megastructures
In Iain M. Banks's novel Look to Windward, an interesting megastructure is featured, but only vaguely described: the airsphere.
Basically, it is two HUGE (on the order of a brown dwarf) spheres stuck together, filled with breathable atmosphere, spinning for gravity. It is surrounded by several "sun-moons," moons with huge spotlights on them to provide light, moving around the airsphere and shining at different intensities, in a chaotic pattern. It moves around the core once every galactic year (forgot how long that is, but its hundreds of millions of years)
Inside, the most important creatures are the Megalithic entities: huge (kilometers long) gasbag creatures that are the equivlant of solid land, great living, moving islands in a sea of air. On their surfaces and inside, they support ecosystems, of symbiotes, parasites, residents, and several races of sapients, sometimes guests from the outside. They are hundreds of millions of years old (the airspheres were built by an Elder Race billions of years ago), and are very open- but inscrutable. Very friendly, but kill one and you'll be hated till the end of time.
The spheres contain other creatures, too, winged and gasbag. They have, at the bottom, a detrius neck, to which falls waste, which is ejected every few millenia, leaving moon-sized droppings, in a ring through the galaxy.
An airsphere can be a fascinating, very alien vista for players to visit, even the setting of a campaign, possibly one involving an empire trying to take over the sphere, versus its biotech-using inhabitants.