Nice breakdown. I'm finding it hard to add six players to this defense, especially as it's been 10 and 9 the last two years. A lot of that will depend on Wilfork, Kelly, Talib and Possibly Gregory though. It becomes a lot easier if none of them are Patriots next year.
There have been a lot of waste on UFAs/trades. Albert Haynesworth, Jonathan Fanene, Adrian Wilson, Shaun Ellis ... not a lot of production out of those guys. Add in some of the more glaring draft misses (Ras-I Dowling, Jermaine Cunningham, likely Tavon Wilson and Jake Bequette) and there's a lot of opportunity to improve the defense that got missed.
I could easily see adding 8 defensive players:
1. A Wilfork replacement, whether a UFA (Linval Joseph) or a rookie (Louis Nix, Justin Ellis)
2. A 3-tech who can provide inside pressure (Aaron Donald, Timmy Jernigan, Ra'Shede Hageman, Dominique Easley, Caraun Reid).
3. A rotational DE who can play meaningful snaps along with Jones and Ninkovich (Michael Johnson, Corey Wootton, etc.).
4. A developmental base DE who takes Bequette's spot (Aaron Lynch, Will Clarke, Taylor Hart, possibly Brent Urban).
5. A hybrid DE/OLB / sub rusher with the athleticism to provide pressure off the edge and play in space (Marcus Smith, Jeremiah Attaochu, Tyler Starr, Ronald Powell).
6. A rangier LB who can cover the middle of the field and pursue, as well as come of the blitz (Ryan Shazier, Kyle Van Noy, Christian Jones).
7. A strong safety complement to Devin McCourty (Calvin Pryor, Deone Buchanan, Kyle Fuller, Antone Exum).
8. A big DB to provide depth outside (Stanley Jean-Baptiste, Kyle Fuller, Antone Exum, Keith McGill, Terrance Mitchell, Pierre Desir, Dontae Johnson, Jonathan Dowling).
I think that it's extremely unlikely that all 8 of these are addressed for 2014, but I could realistically see about 6 of them being addressed between FA and the draft.