I think Marvin Lewis gets a bad wrap, he was all in all a good coach who was severely hamstrung by terribly cheap Ownership. They would never get Lewis any decent free agent signings to work with. He was still able to coach those teams to the playoffs most years, despite Mike Brown's cheapness. Look at how far they have sunk without Marvin.
Marvin Lewis absolutely could be a good Defensive coordinator for someone, and probably even deserves another crack at a Head coaching spot somewhere with better Ownership support. He's certainly more qualified than Stefanski, or an unknown guy from college like Rhule.