Great thread, definitely stirs the imagination.
-In my perfect world, Boise would be roughly the same size as Austin, TX or Nashville, TN to properly fill that gap between Salt Lake and Seattle/Portland. I'd also like to see a lot of the towns dotted around Idaho, like McCall, Lewiston, Idaho Falls, Pocatello, Sun Valley, Salmon and Coeur D'Alene significantly larger than they currently are, ranging from, say, 50,000-300,000.
Here's some more I think would be cool:
-Portland, ME roughly the size of Portland, OR
-Rapid City, SD roughly the size of Des Moines, IA
-Billings and Bozeman, MT roughly the size of Boise and Reno
-I guess technically this is still the U.S. -- it'd be cool if there was another Honolulu-sized metropolis in the middle of the Pacific. I don't know where you'd put it.. maybe a fictional extension of Guam or in the American Samoa.
-This one's kind of weird, but I kind of wish Seattle, Tacoma, Olympia, Bellvue, and Vancouver (BC) were all merged into one city located either where Vancouver or Seattle currently is. Adding the populations together would make it a city roughly the size of Philadelphia... And just imagine what a combined Vancouver/Seattle skyline would look like.