Nice topic. I was actually thinking about the same exact thing a few days ago. Proof of location, theoretically, seems like the only way to prevent centralization, and ensure one vote per entity. Of course the problem then is, how can you prove your location in a manner that can be easily validated. IP address is surely a no go. Also, you have the problem from POS systems, namely nothing at stack to deter a mob from tampering with the blockchain. Granted, there being so many entities on the network, it would be hard to gather a majority for malicious purposes.