flexible block size were proposed, where the maximum weight is adjusted by demand or there are incentives to miners to adjust the blocksize carefully if needed.
This would be better than treating transactions differently, in my opinion. However, it has to be implemented similarly to how XMR does it. Raising the block size would result in a reduction in the block subsidy; otherwise, whatever the max cap is will always be activated by greedy miners. This would lead to larger pools absorbing all available well-paying transactions, leaving smaller transactions with almost nothing.
--snip--
How about BIP 104, 106 and 107? Although personally i'd rather avoid dynamic block size altogether since there's concern it could be manipulated by miner who decide which and how much transaction included on their mined block.