Ringworlds

There are some systems in the galaxy that have only one or two planets and are therefore quite uninteresting for colonization. In such systems Ringworlds could be constructed. The construction of such a massive structure requires a lot of resources and I imagine one would need to dedicate the Industry output of several other systems over several turns to do so (plus a heap of SRs). They then offer space for many Pops and benefit from system improvements just like planets do. In addition, each Ringworld could be upgraded with a limited number of improvements, selected from a pool. So these Ringworlds could be develeoped to serve a certain purpose (defence station, naval yard, tourism center, etc).