Four NFL vacancies have been filled by NFL head coaches that were previously fired from other jobs. Two of them were fired after this season (Stefanski and Harbaugh), one is getting his third bite at the apple (McCarthy) and one his second bite after only having to wait two years (Robert Saleh).
Three additional openings were filled by two NFL DCs (Minter and Hafley) and an NFL OC got promoted (Brady, Bills).
I Don't know how serious Marcus Freeman is about the NFL, but NFL GMs sure seem like they're serious about hiring NFL people to fill their jobs.