The only way to fix the defense is to get two new safeties, two new ILB's, two new DL bodies and a new starting OLB - that should only take us 5 years or so.
Sure you can shoot for the stars, but to have a serviceable defense that can stop the run & get off the field on 3rd downs (and win playoff games) it's more like:
1 new ILB to pair with Williams
1 new NT
1 new safety to pair with Davis
And some added depth at all positions.
That's something that can realistically get done this off-season.
I'm presuming our corners hold up (we have decent depth), that Davis & Dupree take significant strides (and Bud learns to keep his damn trap shut) and Watt continues to improve in his 2nd season. I doubt we could do better than Tuitt & Cam at DE, they just had a weak showing vs. Jags for whatever reason.
And for the love of God, figure out the defensive schemes and coaching hierarchy. Whatever Tomlin and Butler have been experimenting with, pinpoint what works best and roll with it. I'd love to see a new linebacker coach to replace Porter/Olslavksy and a new DB coach instead of Lake, but those moves don't seem likely.
Outside of that, the only other big hole I see is in the return game. Bryant looked pretty sweet running back kicks but they yanked him soon thereafter. Would be nice to have PR/KR who's a legitimate threat, who can be sprinkled on O from time to time.
Anyway, I don't think we're that far off on D. Nor do I think rebuilding this defense is that huge of a challenge this off-season. I wish Colbert the best.