Three initial thoughts:

• I’ve never been one of the Fangio sycophants. I think he’s a good, solid, stable defensive coach but his genius has been overstated for years. The Bears defense took a leap this season because the Bears added one of the two best defensive players in the sport and drafted a star inside linebacker. That’s why sacks, turnovers, performance increased.
• Fangio deserved this opportunity, probably years ago. But Bears fans should get used to this feeling because I predict they will lose multiple coordinators in the Matt Nagy/Mitch Trubisky years to head coaching gigs.  That’s what comes with sustained success.
• This simplest solution is Ed Donatell, who has done a wonderful job with the Bears secondary. But the Bears shouldn’t rush. They need to find a defensive mind that mirrors the folks running their offense: aggressive and exciting. Fangio liked his corners to play off the ball and liked to take his best pass rushes out of the pass rush in big moments. This defense has all the talent in the world. It needs a little meanness.