[Kelly Pedersen]

I'd argue that Space Opera actually doesn't need a specific setting. What it needs is probably a bit more discussion of setting elements then, say, Dungeon Fantasy. But both space opera and space exploration genres are the home of things like single-biome planets and single-culture species. Complex worldbuilding isn't necessary. (There are, of course, some space opera settings that actually do have some pretty intricate worldbuilding - Dune springs to mind - but those, I think, can be left to the individual GM who wants to get complex).

What the book should have, IMO, is some discussion of the various setting elements that commonly go into these genres, and how to use them. Is there an Evil Empire? If so, are the PCs rebels living under its shadow? Or do they live in a still-free society that opposes the Empire? And so on. Make sure to include solid game mechanical details in this kind of discussion ("the Empire is CR 5, which means the following penalties apply to the following skills").

Once you have that kind of detail, I think it's reasonably easy for a GM to assemble the pieces, as it were, to build their own setting, customized to the type of game they want to run.