Clearly, Adalius Thomas is the top choice. The guy is as good a fit for the system as you will ever find. I've come to accept, however, that he will probably sign a monster contract with somebody, and given that he is 30 years old, I'm not sure its wise for the Pats to get in that type of bidding war for him. Sadly, I see the 49ers or Green Bay as more willing to throw boatloads of money at him. I'm thinking a signing bonus in the area of $18 million is not an outrageous prediction, and I don't see the Pats giving up that much.
After that, Jarret Johnson (Thomas' backup in Baltimore, but also a free agent) would be a great addition. He has played sparingly, but all indications are that he can play, and the guy has fantastic size for a linebacker in the 3-4.
London Fletcher-Baker would be a good addition. He is old, but can still play, and Belichick has spoken very highly of him in the past.
Then, like I said before, there is Donnie Edwards from San Diego. His coverage skills would be quite valuable, given that this was clearly exposed as a weakness in our current LB corps. However, he is very small for an ILB in this system, so I'm not sure he'd hold up against the run.