Now that's an interesting idea. I'd wanted to have "proof of decentralization" - require that nodes be physically distant from each other, to prevent mining farms. But I couldn't figure out a way to prevent cheating. It's easy to fake that you are far away, but hard to fake that you're close, because the speed of light limits ping time.

But if you have something that's a useful network in its own right that keeps track of its own coverage, you can use that to enforce decentralization. The higher the density of nodes in an area, the lower the reward.