I think the main problem is lack of talent, no matter who's coaching, and while Pettine might not be a top 8 defensive coordinator, I think he might be a top 16, he just needs some more talent at CB and especially DL.
That being said... if the the Broncos (under the very fire happy Jon Elway) fire Vic Fangio... I'm calling him up and seeing if he's interested in the job.
Gregg Williams might be another interesting name, but at times his personality just rubs me the wrong way... that being said, while his teams have been bad, it's looked like he's gotten more out of the talent than people thought a coach would. Though the Jets haven't won a game this year... of course, that's not all on the defense...
But I don't think just firing Pettine is the answer until you know whom you might get to replace him.
Packers new DB coach has some experience, if they want to fire Pettine and have someone from within run it the rest of the year.
Though, another factor, I think the defensive staff is pretty good... they just need more talent (IMO).