The guide doesn't currently contain any explanation of or guidance on setting and maintaining project roadmaps—though by my count its authors reference the importance of establishing project roadmaps at least 15 times. Because this is a tool most open source projects use (and occasionally misuse), I think the guidebook should include something about it.
I tend to like @funnelfiasco's advice on the topic. With any luck, we may be able to convince him to contribute some of his recent writing for a new chapter or section.