I don't have a dream model in particular, but I do have a couple of dream layouts.
One would be an HO layout based Los Angeles during WWII, running from the San Pedro/Long Beach harbor up to Burbank with a focus on war-related stuff, like the North American Aviation and Lockheed factories, as well as including Union Station, City Hall and some other downtown landmarks, and the huge Southern Pacific yard that used to be between downtown and Burbank. It'd have to be a pretty massive layout, one of things that'd take up 3 whole walls of a 2-car garage or whatever.
The other would be an O gauge British layout set somewhere in the decaying industrial north in the last few years of steam, it'd probably be mostly mainline but with at least a couple of industrial branches that'd have their own locomotives. (Maybe even a separate narrow gauge line alongside the main layout?) I kind of like the idea of it being a garden railway or an indoor/outdoor layout but I'm not sure it'd be practical since I'd want it to be reasonably realistic.