[PvK]

Hmm, shouldn't a ringworld allow planets to be heated by the star, while a sphereworld should more or less ruin all the inhabited planet environments (all become deadly) and their organic resource value?

This would be a reason to build ringworlds rather than sphereworlds.

As for a sphereworld with "windows" - the windows would have to be able to move independantly to track each world.

Maybe suns should only be creatable in sectors with dead suns or system-wide nebulae, and perhaps doing so in a nebula system should create/reveal a few asteroid fields as well. Destroyed black holes could create system nebulae or dead sun plus asteroids. This would make for a kind of cycle effect, where systems could be changed from one type to another and back again.

PvK