I think Mike Pettine could be a feasible replacement. In 5 seasons as a DC, his defenses were never worse than 10th in yardage. Although his defenses did get down to 20th in scoring, part of that was due to the collapse of the Jets as Rex Ryan melted down and the fact that the Jets and Bills offenses were kind of bad. In short, he might be a 'safe play,' if we accept a presumption that switching back to a 4-3 (or a more simplified 3-4 with a lot more base defense) is safe.

An upside to hiring him is that it's not likely he would get plucked as a head coach, since he was generally bad as a HC in Cleveland (who isn't, I know).