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The answer cannot be in the blockchain, because the problem being addressed (resource usage rising too quickly so only people willing to spend tens of thousands of dollars can participate as fully validating nodes) is outside the blockchain.
You will go down the same path as the proof-of-stake folks, coming up with ever more complicated on-blockchain solutions to a problem that fundamentally involves something that is happening outside the blockchain. In this case, real-world CPU and bandwidth growth. In the POS case, proof that some kind of real-world effort was performed.