At the very least fire Nolan and most of the defensive staff and bring in someone like Zimmer, Patricia (if fired) or Quinn. I wouldn't rule out just cleaning house though since it seems like McCarthy is teetering on losing the team, injuries or not.
As far as personnel, clear out as much of the defensive roster as possible. That means Jaylon Smith, Poe, Crawford, Griffen, and most of the secondary. I'd consider re-signing Aldon Smith and maybe Awuzie or Lewis if they're cheap just so we aren't desperate for CB depth. Aside from that, keep only those on rookie deals and those you can't afford to get rid of (Lawrence) and maybe Anthony Brown.
Dedicate as much of the draft as possible to defense and completely ignore offense unless the BPA is a future replacement for Smith at LT and you just can't pass it up, and I truly mean literally ignore every offensive position except for LT. We're almost certainly going to have a pick in the 3-7 range, ideally I'd like to move down for a bounty with a team trading up for a QB but aside from that hope for one of Sewell, Parsons or Rousseau.
I'd like to run it back once more with the offense because I do think we could truly be formidable. The only thing that's stopped us most of the year has been injuries and dumb fuck Elliott fumbling, and that's with Collins and Smith out for most of it, so roll the dice on that improving next year.
And for once, spend some god damn money in free agency, and not on 32 year olds signing for 4/year. Every year there are quality starters in the prime of their careers who can be signed for about 8-10/year, so sign one or two of those.