3-4 back in style again. The Cowboys, Dolphins, 49ers, Browns, and Broncos might all be making switches to the 3-4 defense this year.
Parcells is a 3-4 guy and has wanted to make the switch for a while now, and this year he likely will. He'll target a NT in free agency and work on getting OLBs for the task.
Saban is another 3-4 disciple, and its unsure if they will make the switch but, they could. They just need to get more size at LB. I suspect that the Dolphins are going to purge many of the starters on the defensive side of the ball anyway (as many as 8 could go), and installing the 3-4 with that type of turnover would make sense, since you are rebuilding anyway.
Nolan was converted to 3-4ism in '00 under Al Groh with the Jets. Previously he was a 4-3 guy under Norv Turner and Dan Reeves, but likely will make the switch in San Fran this year. They have the talent at LB with Julian Peterson potentially being a 15-sack guy at OLB.
Crennel will bring the 3-4 to Cleveland. They will have the hardest time adjusting, but guys like Orpheus Roye and Andra Davis should be fine.
The Broncos have indicated they are tinkering with the idea of using the 3-4 more often. It won't become their base defense but they want to use it more in order to confuse offenses. They have the personnel to make it work, despite not having a monster NT (Ellis Johnson can play the spot though).
I think 2 more teams that may make the switch soon are the Giants and Vikings. Both of them have DCs that are 3-4 guys (Tim Lewis & Ted Cottrell). I think the Giants haven't made the switch yet because of Strahan, who I think would be less useful in that system, but when they get rid of him, they can make it work (2006 maybe?). The Vikes have the personnel and a switch would probably help that struggling defense.
So when this year begins, we may see as many as 9 teams (49ers, Browns, Chargers, Cowboys, Dolphins, Patriots, Raiders, Steelers, Texans) using the 3-4 as their base defense, with 2 more (Broncos and Jets) using it a lot. The Ravens are expected to make the switch back to the 4-3 this year, but they still may employ some 3-4 looks as well.
There might be a time soon in the near future where half the league uses the 3-4 scheme (as soon as 5 years?) which is interesting. How long will that last? I think in another 5-10 years, you will see the 3-4 begin to die out again in favor of the 4-3. I think it's more a fad than a long-term trend in the NFL.
This is a reason why I think Matt Stewart is not going to return. His services will be wanted in places like San Diego, Cleveland, and Dallas as they try to make the switch.