I've enjoyed camping in the Mojave National Preserve. Lava tubes, huge sand dunes, tons of joshua trees, and not many people.
Weaverville, CA is a NoCal mountain town that serves as a good jumping off point for hiking, 4-wheeling, fishing, hunting, etc. It's about halfway between Lassen Volcanic National Park and the coast
The first place mentioned in your post, Lake City, Co, has been on my list of places to visit for quite a while. The county in which Lake City is located (Hinsdale County) has fewer than 1,000 residents. Someone once told me that it also has the fewest miles of paved roads of any county in the Lower 48. Don't know if that's true, but if it isn't, it's probably close. I've read about Lake City and heard people talk about it. Some day I will get there.
Other desolate areas that are on my radar:
-- The so-called ION area -- where the Oregon, Idaho and Nevada borders converge. Been kinda fascinated by the idea of checking it out ever since reading "Give a Boy a Gun" by Jack Olsen, about Claude Dallas, the trapper who killed two Idaho game wardens in the early 80's. Some of the most isolated, desolate country in the Lower 48 is in that area.
-- The Steens Mountain Wilderness area of eastern Oregon.
-- Some day I will camp at the Cosmic Campground in western New Mexico when there is no moon, the sky is about as dark as it can get, and I can check out the night sky in all its glory.