Seymour Duncan came pretty close to this idea with their P-Rails pickup, but I want to see it taken one step further: True pickup duality! Instead of using a rail pickup and a p-90, how about making a true single coil that can become a humbucker? A humbucker with a slug coil single coil on top, with 7,000 windings to be a true single coil, and below it on the same pickup a screw-coil single coil with 10,000 turns (or use a stra-bro coil). COIL TAP both pickups, at 5,000 windings, and so you have a TRUE single coil, a single-coil sized p-90, and a TRUE humbucker! The cons of this, is it would have a LOT of lead wires coming off (6?), but the pro would be full tonal capabilities from one pickup. You could wire it to have the single coil, the p-90, and the humbucker (in which both coils are on, but tapped) in series, or the humbucker (in which both coils are on, but tapped) in parallel. You could also wire to have a tapped (at 5,000 windings) single coil, or a tapped p-90.