One of the first things Sean Payton did after being reinstated to the NFL was fire defensive coordinator Steve Spagnuolo. It is hard to say what his reasoning was for the move was, but apparently he didn't like what he saw from afar. There is no reason in the world that the Titans shouldn't fire Jerry Gray and hire Spags, but we already know they won't.
I get the hesitation on a guy like Gregg Williams. He has a lot of baggage and hasn't been that effective for the last few years. There is a lot of risk there, but there is basically none with Spagnuolo. His time as a head coach didn't go well, and last year was a mess in New Orleans, but he has put together some dang good defenses in the past.
Of course all of this is just wishful thinking because Mike Munchak is committed to Jerry Gray, but it sure would be nice.