When I was younger I felt that X-Sector had a space like feel to it, the shapes of the buildings, doors, pipes and the colours used. I know now it's this secret military testing base or w/e, but as a kid the silver and black colour scheme of the rides screamed 'space'. Green camo was military to me.
I'm not saying this is why we should have a space theme, I just mean I never came off Black Hole and thought 'Well that was a bit out of place'. It just seemed to fit in, which is why I think a space related ride wouldn't be straying too far from the current X-Sector flavour, for want of a better word.
I'm not suggesting in your face space shuttles, astronauts, aliens, planets, asteroids etc, something a little more subtle. I admit the space dogfight was a bit out there, was just a quick suggestion for the duelling element.