There are a ton of places to do this in Colorado. Every year my buddy and I choose a high mountain lake to backpack into and every year I get more frustrated that I wouldn't be able to hit all the spots we want to if I had 2 lifetimes. Don't restrict yourself to 100% off trail all the time. There are a bunch of places where you can start on a well used trail branch off to a little used trail and finish up with some bushwacking. We hit a lake a couple years ago that has a trail for much of it, but then fizzles out and you try to stay on elk trails the rest of the way. I doubt the lake sees more than a half dozen people all year if that.
The collegiates, san juans, holy cross wilderness, grand mesa, flattops, etc all have places you can disappear.