Safety - The Pats safeties are actually a good group. Definitely the 2nd to last place I'd spend money or a draft pick for the Pats Defense with LB being the last.
DE - A group of Chandler Jones, Michael Buchanon, Rob Ninkovich, and Andre Carter is actually a good group.
DT/NT - This is where the Pats need to focus, imho. They desperately need someone who can start to take over for Vince Wilfork at NT. I think that Chris Jones makes a great 3-Tech guy next to the NT on the left side. Siliga, imho, makes a great reserve NT, but I don't see him as 3-Tech guy or a starting NT. Vellano is probably best as a reserve 3-Tech guy.
CB - The Pats need someone to replace Talib in terms of depth. I honesly believe that Ryan and Dennard have the ability to be staring corners in the league. People forget that Ryan is a rookie and that Dennard missed a bunch of time with injuries. The next CB has to be someone who can play OUTSIDE, not at the Star. The Pats need to keep Arrington in the slot because he's so horrible outside.