It would not surprise me if we use a base 4-3, like always, but use the 3-4 in certain situations. Although I worry about linebacker depth and who would be a true nose tackle. It's also easy to get a 3-4 confused with a 'Spinner' package, which is essetnailly the same 'look' but with different personel and pressure/schemes.
We have NEVER ran a 4-2-5 under Bo, with the exception of when we played @ Texas Tech in 2008 and lost in over-time. That was a 'true' 4-2-5. A 'true' 4-2-5 consists of: 4 down linemen, 2 linebackers, 2 corner-backs, 1 free safety and 2 strong safeties. The Strong Safeties are near the line of scrimmage next to the linebackers and align outside of them near the line of scrimmage and the Free Safety is the ONLY safety in the backfield. A single high look.
Bo likes to keep the consistent two high look: 4-3, Spinner, Dime, Peso, Nickel, 3-4, etc. Very rarely do we go with a Single high look. But all coaches can adapt new philosophies so we will see.
A 4-2-5 would get killed by a strong running attack. Although there are exceptions [TCU vs. Wisconsin], that is generally not something you would want to run against a bigger powreful running team. A 4-2-5 is great against spread/passing teams as a base though which is why we ran it against Tech in 2008. Our personel for that game was probably better than anything else we had.
The Peso defense we ran in 2010, was a nickname speically for Eric Hagg who was the Peso Back, but it was a Nickel package designed around Hagg's strengths. Peso is just another word for Nickel with the exception that the Nickle Back was not an oridnary Nickel Back, Hagg could do it all. You could argue Hagg played like a strong saftey and linebacker all in one, and he was a heck of a player for the scheme Bo and co. built around him.
Bo likes the word Multiple. He said that he wanted to be more multiple on defense last year but couldn't be. With the secondary improved a lot, or so we've heard, I expect to see different packages for different situations/downs/teams.
We might see a 3-4 in a passing down where we need some heat, or maybe even a Spinner package against Ohio State or even a 5-2 against Wisconsin. Okay probably not a 5-2, but I would like to see that scheme go up against Wisconsin.
The bottom line is: Bo was multiplicity. We might see it this year on defense. More blitizng, different packages, better rotations and more depth. At least that's what I hope for.