Colorado
Everything that has been said about Colorado here applies primarily to the front range. It also applies to EVERY other major population center in Utah, Oregon, Washington, California, and yes, even Europe, although the Europeans are generally less prone to uncontrolled sprawl.
Like it or not, the reality is that this will not get done without Lance's backing and Lance will want it in his new home of Colorado, so sorry to the people in the NW but your out of luck in this one.
The key is to keep it to western Colorado where there are lots of roads with very little traffic. I like the idea of adding onto the Tour of Utah. You could easy plan a route from Salt Lake over the wasatch, along the southern edge of the Uintah's, into colorado near rangely, down to fruita, over to Aspen (gotta make Lance happy) maybe include the road from Grand Junction to Naturita, over to Moab, the La Sal loop road, up through central utah and back to Salt Lake. These roads would have very little traffic, especially in the fall (the only reasonable time of year if you want both mountains and desert). Also the towns in this area have an ungodly number of empty hotel rooms between summer and ski season and will quite frankly beg for the business in the fall. They are all also used to large cycling groups coming through with RTR, BToCO, etc. Early October during the peak of the Aspen color would be ideal, it would show the world the beauty and diversity (mountains, forest, desert, etc) of this area that is unrivaled anywhere.
TFF, this is my very first post but I've been reading yours for a long time. You have a right to your opinion. So do I. In my opinion western CO and eastern UT blow the doors off anywhere in the cascades for something like this. The cascades have almost no roads above or below timberline and thus TV coverage would show nothing but trees. Its beautiful but not nearly as diverse and without the wide open vistas that make TV coverage of the european tours so beatiful. You have too many trees!
I don't think the distance from population centers will be a problem. Bring a world class field to a world class course and the world will show up. None of the area I've mentioned is more than 5 or 6 hours from either Denver or Salt Lake... How many people live within 3 hours of Alp de Huez?