Don't know if this has been suggested already but would it possible to add Niven rings ("halos")and Dyson spheres to the game?
a Ringworld would be built over a sun and would have something like 3 times more space than a planet (max size planet ofc) and player could select it's enviroment type.
a Dyson sphere is almost same as the ring but it would have, hmm, 9 times more space as a planet and it would freeze other planets (if any) because it's blocking the sun...
The rings and sphere techs would be in the end of the tech tree and build them would require enormous amounts of money and stuff. And there should be limit how many you can build. Maybe one ring and then you could convert it to spehere.
Or could this modded to the game?