Dynamic coinbase as incentive to fill blocks
This adds an incentive to fill blocks with ANYTHING that will take up space in the block. There are many different ways to incentivize including transactions, but they can typically be gamed because miners can create transactions themselves, and they gain all the fees from those transactions right back into their coinbase transaction.

Your suggestion, which has good intent, would likely cause miners to be incentivized to fill their blocks with meaningless transactions, and increase the already ever-increasing size of the blockchain, making it harder to run a full node.