I would certainly be ok with retaining Fangio. However,
I wouldn't keep Fox in order to retain Fangio.
I wouldn't promote Fangio to HC in order to keep him.
I would prefer (and expect) they hire an offensive-minded coach, and hopefully get a very good DC. And I hope they don't flip back to a 4-3 and start over.
I know this is not always how it works, but in general, I believe if you hire a defensive minded head coach, he meddles and wants the offense to be conservative.
If you hire an offensive HC, they tend to turn the D over to the coordinator, and don't try to run it.