I think you have it right on. Getting more of the security guards, end users, etc would be great. My opinion on the manufacturers side is to talk to the Sales Engineers rather than the sales managers/reps. Installers and reseller/integrators sales people is a great idea too.
Not so much a "who", but for some of these it would be cool to see you interview for example a security guard from a large school and a small school, a loss control person from a clothing chain and a hardware store. I think it would be valuable to see where certain products, procedures, etc. are commonplace across different iterations of the same vertical, and where things are done differently based on facility type, location, size, etc.
I would be interested in interviews of major universities director of public safety. With huge campus environments they get a wide variety of security concerns whether its rapists or college shooters. Would be awesome to get insight into those people's criteria for security equipment.
I've always been curious about how the end users really feel their system works and meets their needs after an install. I always see those case studies, and a lot of them almost look like they were writting by the manufacturerer and then filled in with customer information.
End uders/Security Guards, and so on.
Thanks all. We are definitely going to go down that path, including schools and other 'applications'/verticals.
Any more specific ideas - keep them coming?