The 3-4 defense sounds good because Von Miller and Elvis Dumervil could wreak havoc as outside linebackers while the bigger, slower veterans in Joe Mays and Keith Brooking man the inside position. Ty Warren, Derek Wolfe and Justin Bannan would be the three-down linemen.
It sounds good, but it's a horrible idea. Changing the defensive scheme does not fix problems with gap integrity and tackling, it exacerbates it. Switching to the 3-4 does not suddenly make a player a better pass-rusher.
The defense scheme merely puts the players into the best possible position to be successful. Most of this isn't by making sure the best players can rack up the stats, but that they can cover the deficiencies of other players. It's a team concept.
Let's assume for a second a switch to the 3-4 this late in the preseason would even be possible, which players would benefit and which players would struggle?
Justin Bannan was one of the league's best run defenders at defensive tackle last year. According to ProFootballFocus, he was ranked 16th in run-stop percentage, which measures the number of snaps the defender had on running plays versus the amount of stops they recorded. Stops constitute an offensive failure.
The nose tackle in the 3-4 is one of the most demanding positions in all of football and the key to running the formation as the base defense. Without a good nose tackle, the 3-4 can be nothing more than something used on passing downs. Does that fix the Broncos' issues with run defense?
If the nose tackle in the 3-4 doesn't successfully draw two blockers and fill two gaps, either Joe Mays or Keith Brooking will have to shed an offensive lineman to make the tackle, or a gap will be totally uncovered.
Bannan will play the 4-3 nose tackle spot and will be counted on to draw two blockers, but not to man two gaps. Could it be Bannan is struggling and that's making everyone's job in run defense harder? There is no reason to give him more responsibly at this point and making him a 3-4 nose tackle would just weaken his impact.
Ty Warren was a defensive end in New England's 3-4 and hasn't played in two seasons. He's a classic 3-4 DE, but many bigger 3-4 defensive ends find the transition to the three-technique defensive tackle rather easy. For a similar example you need look no further than Richard Seymour in Oakland.
Derek Wolfe is playing the role of a 4-3 power end and could easily translate inside to the three-technique defensive tackle or to a 3-4 defensive end.
Warren and Wolfe are good fits in their current positions or in the 3-4, but switching to the 3-4 base defense would put more pressure on Bannan and actually hurt the run defense.
Von Miller and Elvis Dumervil would be the outside linebackers. Sure, Dumervil had more sacks as a 3-4 OLB, but his run defense was also a lot worse.
ProFootballFocus gave Dumervil a +7.3 grade in 2011 at defensive end against the run and a -12.1 grade as an outside linebacker in 2009. What's the problem with the defense again? Run defense or pass rush?
What was wrong with Von Miler in the 4-3? He got high marks in the pass rush and run defense and the Broncos rarely asked him to cover. What exactly would the 3-4 do to help Miller? His role wouldn't change at all.
Joe Mays and Keith Brooking would be the inside linebackers. I guess that means Wesley Woodyard is back being the nickel linebacker. It makes sense on the surface, but when you factor in that the best cover linebacker Woodyard would be on the bench in the base alignment, you wonder if having the thumpers up the middle really help.
Hybridization of the NFL
The 3-4 does nothing more than put more responsibly on Bannan and take Woodyard off the field in favor of Brooking. It really comes down to if you would rather have Woodyard or Brooking in the base defense.
When and if D.J. Williams comes back and the Broncos find a 3-4 nose tackle, the 3-4 might be a good idea, but until then it's simply not ideal to further complicate the defense in the first year under yet another defensive coordinator.
In the end, the defenses in the NFL can only be described as hybrids as they are usually designed to limit an opponent in one particular week or situation.
Suggesting the 3-4 defense would somehow fix what ails the defense in Denver is just a fantasy.