[Baron Munchausen]

Theoretically, you could just put 'ringworld' or 'sphereworld' into a new system in SystemTypes.txt and this new system would be generated in the usual way according to how you setup the QuadrantTypes.txt file. The problem would come when the empires were placed. If you were just relying on random placement there would be no guarantee that a ringworld or sphereworld would be selected.

And even when it was selected the way the creation of a homworld works is hard-coded (will not respond to the AI file settings) and you'd end up with the default number of research and other facilities and 100+ mineral miner facilities. If you started with some good organics/radioactives planets nearby I suppose you could scramble to get those started and start to exploit your resources but it's still a bit clunky.

You could design a map with deliberately placed ring/sphere worlds and set starting points on them to insure that empires were placed in them. This doesn't fix the above problem, though.