There was a paper last week pointing out how cost effective 51% attacks are on the lesser cryptocurrencies.

Proof of work has become very expensive. Proof of stake has another set of problems.

How about proof of geographical distribution? If each node had to be a minimum physical distance from another node, that would put a big crimp into mining farms.

Distance can be verified. You can use speed of light lag to verify that two nodes are not further apart than some distance. So set up a mesh network. If you claim to be at a location, others in the mesh can verify that it's roughly correct by pinging it and measuring the round trip time. You can fake a longer distance, but not a shorter one. You might be able to do something in the WiFi bands in the 100m - 1KM range. Require that each node be at least a few hundred meters from any other node. Give an advantage to suburbs and rural areas.

"HeartlandCoin"?