Military Space Projects is definitely begging for an update. Lots of developments since 2008. Not the least of which is the creation of the USSF. Not to mention developments from Russia, China, and other space powers. As well as private corporations like SpaceX launching military payloads. Definitely room for an updated volume.
Difficulty: how much has actually come out of the black world beyond speculation? There might well be a bagrillion neato space weapon projects since 2008, but if they're all classified, there's not going to be much to write about.