Proposed is an item that should be built-in to every digital camera.
On all modern cameras, everything is done through the lens. This is great for metering exposure, but is a poor choice for white balance, especially on longer lenses with a narrow field of view. The only reliable way of setting white balance is to take a picture of a grey card, which reflects all the incident light evenly. In effect, this averages light from all directions.
So, fit an auxiliary sensor to the top of the camera. A little plastic dome which accepts light from all angles, like a tiny fisheye. This should not cost more than the price of a cheap webcam, but would function like having a grey card reading for every picture.
Pedantry: Actually, for full solid angle, you would need one fitted to the bottom as well. This is unlikely to be as useful. (Except maybe for portrait/upside down shots?)