To those poking fun at looking back to the past, here is the basis for my suggestion: 1) it would be hard to find someone with a better resume than Hankwitz, and 2) he might want / be willing to come back.
In support of claim #1, in addition to being DC for our National Championship team, he has been DC for great defenses at Kansas, Texas A&M, Arizona, Wisconsin, and now Northwestern. That's right, 24 years being a full-fledged DC. That addresses all the folks here griping about lack of experience in the position being hired for.
In support of claim #2, he spent 12 years in Boulder and had a lot of success here. Given that, I'm guessing he may have a soft spot for this program. It is hard to spend a decade in Boulder and not think about returning, especially from Chicago. Boulder would certainly be a better place to retire. Also, odds are he is not making a mint at Northwestern, so we might be able to compete salary-wise. Also, other than the 10 year stint at CU, he has bounced around a bit, so it doesn't appear he has massive loyalty to one program.
So, I would really be interested in hearing someone else make a reasonable suggestion for a new defensive coordinator, with your reasons. Fire away...