I honestly think the HC choice is (or at least should be) the second most important decision going into next year.
What I don't believe we've ever had here is a GM who has both the autonomy and the stones to shape the roster the way it should be shaped. Casserly had the authority, but all too often acquiesced to the coaching staff with horrible results (see Patrick Buchanon, and Jason Babin). Rick Smith appears to be little more than a yes-man for Kubiak.
The teams everyone points to as the immediate turnarounds (Falcons, Ravens, last years Dolphins, this year's Broncos) are all coached by first-time Head Coaches, but also have GM's/FO's that are well managed, and autonomous of the HC. Denver may be the exception to this (I'm not sure), but even with them, they have a GM who has a background that doesn't suggest he is there simply to agree with everything McDaniel says.
I honestly still believe there's a lot to like about Kubiak as a HC, but it appears his job goes well beyond HC, and what I don't think ever happens is a challenge to his opinion. While it's apples/oranges in terms of achievements and success, I think the same thing happened to Rudy T with the Rockets. Nobody ever thought to challenge his decisions, they just carried them out (can you say Kelvin Cato). It's been very rare in the NFL that somebody with both HC and GM responsibilities succeeded. Mike Holmgren couldn't, Bill Parcells has had his greatest success wearing just one hat, and most of the NFL's model franchises have strong and autonomous front offices.
Go out and find the best GM you can get. Doesn't matter whether it's a proven commodity, or a star on the rise, but get somebody good, give him the authority he needs, and work on who the coach is after that.