Assuming everyone comes back healthy, I don't think you need anymore defensive additions.
DL: Hutch, McNeill, Reader, Smith
LB: Barnes, Campbell, Anzalone
DB: Davis, Branch, Joseph, Arnold
Plug in those four players and you'll get much more QB pressure, much better WR defense, and better run defense as well.
Obviously you always try to upgrade, but this defense is not a bad defense when healthy. Just way too many injuries to over come.