Without seeing which head coaches will get fired and which DC's from fired staffs will be available, I'd say my two choices for defensive coordinator would be:
Brent Venables, age 46. Clemson's defense is always elite. Plus, I know him so that would be cool.
Michael Barrow, age 47. Onetime Dallas Cowboy, he currently holds the title of Assistant Head Coach/Linebackers for the Seahawks. He has coached at the U of Miami and the Seahawks for the past 11 years. Seahawks promoted him to "Assistant Head Coach" because they have Kris Richard at DC, and didn't want to lose Barrow.
If money or contract status were not an issue, I'd want Gus Bradley (age 51, just took Chargers DC job) and then previously mentioned Kris Richard (age 37, Seahawks DC).