A dominant defense is a pretty damn hard thing to bank on, no matter what you get in a trade. Injuries, salary and cap space play a significant role in keeping a "dominant" defense on the field for an extended period. With the continuously shifting rules towards offense, it makes it even harder.
Could you name me a trade that you think would give us a high probability of a dominant defense with Cassel for multiple seasons, that is also likely from the opposing team's POV.
If a trade was made, the team which seems to be the best candidate is the 49er's. Another member here, who's football opinion I respect, mentioned LB Patrick Willis (6'1", 240, 23 years old, $2,326,000). The 49er's then trade their #1 and #2 to another team in the top 7 of the draft. Thus, I believe the 49er's become instant contenders without stripping their team and Brady is back home (whatever).
The Patriots then take that #1 and deal it for 2 higher 1st round choices, or they can trade for a vet and a pick. It's too early to determine the pecking order of the college class. So lets say a team with multiple needs will trade out ( Bengals, Lions, Seattle, Raiders, Browns, Rams, and Kansas City).
I have confidence in the Pats first round selections over the years to secure two high caliber defensive players. But just for fun, lets trade a #1 to the Raiders for CB Asomugha (6'0, 225, 25 years old).
With my other #1 I'd take a safety.
Results: Defensive line (check) Linebackers (check) Corners (check) safeties (check). Nothing is a sure thing, but having these athletic defensive members would make a huge impact.
Cassel is not Tom Brady, but he is going to be damn good. Considering his college and pro career, for all intents and purposes, this is Cassel's rookie year.
Again, I'm not saying do this (no one in this thread has said the Patriots SHOULD make the trade) just saying what if.