Y'all can laugh, but it's too bad he's too old... Otherwise I'd bring back uncle Wade. Heh heh and all that, but he's one of the best DCs who ever lived. He'd have Micah at or breaking the all time single season sack record, and I'm not even kidding.
But I'm with Cotton, I'm not so sure Quinn is leaving anyway.
I hate to tell you guys but I think this really is it for Quinn, Seattle is the job he's been waiting on in my opinion.
The last two years I consistently said I thought there was a much better chance of him returning when most thought he was a lock to leave, but I think he's starting to sense that he's maxed out here with some of the same patterns playing out year to year (boom/bust style of play, randomly getting gashed by the run every 4-5 games, etc.). That's not to say I think we'd decline if he stayed, we'd still have a very good defense, but I think he's sensing it's time and Seattle has likely always been his top choice given his familiarity with the front office and ownership.
With all that said, now that McCarthy has really proven himself (in my eyes at least, pending what happens in the playoffs), I don't think Quinn leaving is as much of a concern as it would've been a year or two ago. There are usually strong DC candidates who match Quinn's profile that are available each year, guys like Wilks, Frazier, Gus Bradley and so forth, with several coming next year out of the Saleh/Eberflus/Allen/Bowles group.
Quinn is a great DC and I'd love to keep him, but it's not worth making him HC anymore and there will be others like him available, we just have to hope the organization goes after them when the timing is appropriate.