I kinda liked your proposal at first glance, but then I realized it is already implemented.

You see block size IS already "dynamic". Code only specifies maximum block size, so as a miner you can generate blocks of any length up to that size. Theoretically you could make limitless block size and let single miner empty the mempool in next block but that will end up in blockchain spam and insane chain bloating (up to the point bitcoin becomes absoletely unusable). Therefore you need to specify certain reasonable limit for each block. And that's what has been done. Some people argue that this limit should be higher, and by all means it can be higher but that isn't the solution since bigger blocks means bigger blockchain, bandwidth usage and bloat. Lightning (perhaps not in it's current state) is completely different - it allows to send barrages of valid transactions without actually posting most of them in blockchain, so blockchain will remain it's function of "bank", keeping your money safe, while lightning will allow you to use your money effectively in small chunks.