Before reading any responses, I figured I'd post first so that I wasn't influenced in any way...
At the start of the year, I said RT was our biggest upgrade necessary across the OL. But having now played over half of the season, I think an argument can be made for either guard spot over RT, as porous as Polumbus has played. And along the same lines, we need a nose tackle.
Barry Cofield has done as good of a job as he can, and he is a skilled football player. However, he would probably better be suited at end for us and in a rotation. One of the age old adages when it comes to playing an odd front defense is the statement that you need a big, powerful gap plugger in the middle to make the defense work. Yes, the advent of the 1-gap 3-4 by Bum Philips and Wade Philips has given some flexibility at nose, but even then you need a guy who can plug a hole and generate a push. It ALL starts in the middle on both sides of the football, and Barry Cofield is not a nose that you'd need to make the defense flourish. Having said that, I really do believe he is a valuable part of the team, and him being able to play end and plug in at nose in spell duty is a HUGE value. But we need that monster.
Kansas City started rookie Dontari Poe at nose last year. He had a decent season but nothing to write home about. He had one offseason in the NFL and the Kansas City defense is thriving with him in the middle. Justin Houston and Tamba Hali are studs off the edge. And yes, both are tremendous pass rushers, and it's quite possible both of them are better than either Rak or Kerrigan. Having said that, if there was an interior push in the middle, Kerrigan and Rak become THAT much more deadly. I've said it for years, along with offensive line, we NEED a nose if we're going to stick with a 3-4 (and while we're at it, let's get a new coordinator to go with the new nose!)
We'll also need an OLB. Rob Jackson and Brian Orakpo are both free agents. We'll need to make some tough decisions monetarily in that department. I'd like to see both back for reasonable contracts, but I don't know if that will happen.
I also think we need one new safety. Rambo will improve over the offseason and Philip Thomas will return. We still have Reed who can be serviceable and perhaps we have Meriweather. Bring in one more safety, trim the fat and see what we have.
And of course, at least one ILB will be a necessity.
Could we upgrade WR? Sure. But I don't think it's as pressing.