I'd go after Kevin Greene, outside linebackers coach for the Green Bay Packers. He's currently coaching in a 3-4 but we throw a lot of 3-4 wrinkles into our defense which is barely something you could call a base 4-3 anyhow. Greene also played in both 3-4 and 4-3 fronts, including time as a 4-3 OLB with significant pass rush production, so he'd understand Von Miller's situation quite well.
He hasn't been linked with any DC jobs despite the success of Green Bay's LBs, likely because the defense as a whole has been suspect and I'm sure he's quite picky about not getting to coach a talent on par with Clay Matthews. Von Miller offers an elite alternative with Doom as icing on the cake from a "look what you have to work with" perspective.
He'd be able to take this system and keep it going. Lovie Smith and Rod Marinelli would turn it into a much more conventional Tampa 2. I don't think we have anyone on the staff now with enough time under their belt to be given the reigns completely and Fox doesn't have the time to pick up the slack himself.
It's all moot though because I don't think JDR is going anywhere. McCoy looks like a shoe in to depart but that is a vacancy you can fill well without a ton of fight.