I believe we're going to see nickel end up being the base defense this year with more zone and 3-4 looks than usual. Jim Johnson has a lot of personnel to work with here.
I figure that the Eagles will keep 10 defensive linemen (Trent Cole, Juqua Parker, Victor Abiamiri, Bryan Smith, Darren Howard, Mike Patterson, Brodrick Bunkley, Trevor Laws, Kimo Von Oelhoffen, Montae Reagor) who will all see the field.
The starters at linebacker are set with Gocong, Bradley, and Gaither. Akeem Jordan is able to play in the event of injury, or in a 3-4 look. Moving Gaither to the WILL position could conceivably result in him picking up 200 tackles (he had 170 at middle last year, and having him on the edge could really utilize his speed well, especially in 3-4 looks).
They have three starters at corner with a very good dime corner in Joselio Hanson. The safety position has a healthy, veteran Brian Dawkins returning. Quintin Mikell is beginning his second year as a starter. The strength of the Eagles defensive backfield matches up well against the strength of the NFC East passing teams.
In all, I think this defense will be one of the top three defenses in the league with San Diego and Minnesota. They just look nasty, and much faster than I can ever remember.
Jim Johnson has a lot to work with this year; this could be a very exotic, intricate defense which should give offensive coordinators headaches all year.