This is a wonderful suggestion, and it would be the best use of Gatan material as presented in OQ2 if it's not to be expanded upon in the foreseeable future. A sixty-four page long gazetteer would make the core rulebook much heavier than it needs to be, while twenty - thirty pages don't offer enough meat to serve as a campaign setting. But those same twenty pages would serve as an excellent example in a book on worldbuilding.
I agree that it's vital to keep the project managable. Some polish, slight changes and revisions, new artwork and improved layout, all of this is already something that makes a great game even better. But it would be great to hear about a vision for the product line, even if it's stretched over the next decade. I'm pretty sure that I will still be playing OQ those ten years from now.
The 20/40 modifier rule is already something I use at the table. Simple math is something that sits well with the idea of OQ being a simple system.