Just looking at google maps, I have always wanted to do a continental divide from Canada to Mexico. You could even shift that north and do a "from phat's beloved Jasper to Denver". Or you can be more creative and do a Montana, Wyoming, Idaho, Utah, Arizona (with a detour through Vegas) trip. Or try a historic western migration route (like the Oregon trail from Missouri, or the pony express, or Mormon trail from Illinois). Just some ideas.
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