A GPU could be a bit tricky because, most of the complex airplanes that most people want to use, either already have it (like the PMDG) or have custom electrical systems anyway so, it would be very difficult to make it work with every airplane.
Keeping the airplane powered up, would require *writing* into the airplane data, which is potentially dangerous (is not as safe as just *reading*, like we do for custom doors) and might require reverse-engineering every complex 3rd party airplane out there. And if we did a GPU that would work only with default airplanes or airplanes with the standard FSX electrical system, people will complain they want to use it with that xxx 3rd party airplane, and taking valuable parking space to make a dummy/fake GPU that doesn't do anything, it's not a very good idea either.