Which new player? For me, if I had to chose ONE player, it is all about the defensive side of the ball and that guy is Jamie Collins. I want him to be the bookend pass rushing and coverage linebacker that this team needs against playoff quality offenses and this year, the Pats are facing 8 teams that have dynamic running back and tight end sets.
Collins can help with the pass rush and can certainly help in coverage.
The next guy is Mr. Armstead. He can provide that 2nd and 3rd defensive tackle to the rotation alongside Wilfork and hopefully Kelly, then their interior pass rush and ability to limit the inside run is only going to help their defense become one dimensional.
There are plenty of candidates on the offensive side of the ball with Amendola, Dobson, Sudfield, Zusevics (I am counting him because he was a practice squad guy), Boyce and Leon Washington.
With that said - I want this defense to be feared by opposing offenses. The defense showed enough progress in the 2nd half of last year that it was getting real good following the Talib acquisition that with the addition of the current personnel that they could take a step forward as a true top 10 defense in the NFL.