As far as I understand your principle, you propose to regulate the transaction fee through changing the block size.
I suggest another option: adjust the block size by changing the transaction fee.
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Disadvantages:
1. Need to change Bitcoin protocol to explicitly disallow transaction low fees. Currently minimum fees is only set with parameter
minrelayfee, but miner could include transaction with 0 fees on mined block.
2. Miners could manipulate minimum fee simply by including their own transaction when all transaction on mempool isn't enough to fill the block.
The main idea is really not bad but I think we'd have a hard time to get everyone to agree on those terms...
That apply to all changes which require hard-fork