Who pays for congestion? Optimal design of protocol fees

Not linked yet here, but definitely related is Make EIP 1559 more like an AMM curve .

Good point. I think we should go further and make only the users of the very same block pay more, and not the users of future blocks. This means that as soon as a demand spike subsides, fees go back to normal levels and users do not need to wait for the fees to go down.

What is “normal level”? How is that level determined in a way that cannot be manipulated?