Assuming we hire a young offensive coach, we will need an experienced defensive play caller - which will consist of fired HCs and DCs (likely won’t have an up and coming DC. A lot of fired HCs end up as defensive coordinators: Dan Quinn, Brian Flores, Vic Fangio, Rasheed Morris (interim), Steve Wilks (interim), Jack Del Rio (lol).
Candidates: Belichick (no shot), Brandon Staley (runs a 3-4), Ron Rivera (intriguing), Todd Bowles (not sure he gets fired), Jack Del Rio (lol)
Personally, I don’t like any of the options you mentioned either than Bill or Bowles, and I don’t think either of those happens. Staley’s defense has been an absolute failure with the Chargers, del Rio’s defense has hardly worked ever since he took the Raiders HC job, and Rivera seems out of touch with the way the game has been changing (which is pretty evident when you watch his press conferences). I think our best bet really is to take an assistant from a defense that has been succeeding for multiple years.