Perry Fewell is the best candidate available.
Aggressive 4-3 style, attacking defense. He even did a decent job when he was the interim head coach with Buffalo a few years ago.
We'd have to fire Pioli because he would never bring in a 4-3 coach, but firing Pioli should be a given anyway.
The answer is Perry Fewell.
If you can't draw a crowd, draw dicks on the wall.