Jetrock
Mongoose
One difference between a space-opera SF game setting and a fantasy game setting is typically one of scale: fantasy games are often set on a single world or a single continent (aside from things like Spelljammer, which are basically SF games with fantasy trappings, or games/campaigns set on multiple planes), while the typical SF campaign (especially in Traveller) has a setting consisting of hundreds or thousands of worlds--so often, a single world in an SF setting is the equivalent of a city in a fantasy setting (and often players' experience of a world is limited to one city, generally the starport.) There have been quite a few Traveller supplements that did a deep dive on a particular system or world (Beltstrike! and Tarsus from the CT era, the GURPS Traveller worldbooks, etc.) based on the idea of an extended campaign in a single star system, but typically in that sort of campaign there's still likely to be a single city serving as a "home base" of sorts, which will have regular hangouts, NPCs, and other features of a "limited to one city" campaign. And, as another poster already pointed out, the Chaosium Sanctuary set included Traveller stats--and considering the wide variety of tech levels and cultures in Traveller, the idea of a medieval-era campaign game (with no pesky Prime Directive to worry about) could be a lot of fun in the hands of the right referee--and barbarian/drifter/other low-tech PCs would be in their element!
