Mining pools were formed quite early in Bitcoin's history. Since then, they have not been considered ideal because of more and more centralization creeping in. But it seemed the community has learned to accept and live with this necessary evil.
So in this context, has it ever been discussed to build mining reward pooling into the actual Bitcoin protocol, making centrally administrated mining pools obsolete?
A technical implementation could look like this:
A miner that finds a hash with difficulty n still solves a block around every 10 minutes, but also miners who find a hash with difficulty n-1, n-2, or even n-3 can submit their solution, however without triggering a solution for the current block.
The mining reward could then be distributed in one transaction proportionally to all miners who have found hashes.
Are there any altcoins that implement this or a similar concept already?
I'm aware there's P2Pool, but it uses a separate blockchain, which causes latency issues that make it unable to compete with centralized mining pools.
 Submit their solution how and to where? This would be a technical detail to discuss later. They could be opening a transaction channel or the like.