I completely agree, improving the pass defence isn't about adding 1 or 2 pass rushers and looking for a quick fix, its about improving the defence as a whole, which will in turn improve our pass defence. It all starts by stopping the run. By stopping the run we are able to then be more creative and aggressive with our blitzing, and use more resources trying to stop the passing game. I think the best way to go about this is by adding an athletic, run stuffing RDE, and a strong edge setting OLB, that is also a threat to rush the passer. This not only improves the run defence, but allows our LB's more freedom to make plays. I don't believe a specialist pass rusher or adding another DB will have as much of an impact as a good all around DE and OLB.
While I do not disagree with this general assessment, I think it's a little too simplistic of an answer. IMO, there are 2 areas that need to be improved - base defense and 3rd down.
In the base, there are too many "specialists" - guys who are only good at stopping the run (Vince, Brace, T Warren, Spikes, Cunningham), or only good against the pass (TBC, Wright, Guyton). So adding a complete player at either DE or OLB position (or ideally both) would improve the situation. This has to be priority #1 for this team.
On 3rd downs it's pretty straightforward - Pryor needs to be improved on, Wright has been hurt and may retire, and the LDE spot - which has been occupied by the likes of Crable, Burgess and Moore - needs to be improved on. Of course, if you improve the base defense by adding good 3-down players, you improve the 3rd down D as well. But in any case, the 3rd down pass rush has to be improved.
PS: to answer the question of "who", I would go like this:
Jordan, Watt, Heyward, Fairley and Wilkerson could all project as 3 down DEs for the Pats, though some project more favorably than others.
Von Miller and Ayers project as 3-down OLBs.
Everyone else, IMO, is either a 2-down run stuffer or a 3rd down pass rush specialist.