I'd like to see more derelict stuff... wreckage of destroyed stations and capital ships. Everything we see is too new and shiny. If mankind has expanded as far into space as the game represents, then there would be a lot of older, abandoned or destroyed stuff left lying around. It wouldn't be just scenery either, there could be a chance to find artifacts or other loot floating nearby.
Another thing I'd love to see, although the game engine probably isn't up to it, is a massively populated orbital ring of separate space stations. Something like the Glitter Band in Reynold's space opera novels, with 10,000 small orbital habitats ringing a planet. The idea that the more populated systems would only have one big station orbiting around each planet seems very unlikely.
While the idea is cool in the abstract, I wouldn't want to see Ringworlds and Dyson spheres. That implies a level of tech that humans don't show in the current game, and I wouldn't want to meet up with whoever was making them. The game engine probably isn't up to showing something that size anyway.