The NFL coaching market is as captivating as ever — and could become even more compelling. Seven teams need to fill a head coaching vacancy, while several of the NFL’s premier franchises may soon join them. Mike McCarthy is firmly on the hot seat after his latest postseason disappointment with the Dallas Cowboys, while the Philadelphia Eagles have a choice to make about Nick Sirianni following a late-season freefall and first-round playoff exit. Even the future of Mike Tomlin, who has never finished with a losing record in his 17 seasons with the Pittsburgh Steelers, became the subject of spec…