One solution for this already exists: P2Pool. From the wiki page, "P2Pool is a decentralized Bitcoin mining pool that works by creating a peer-to-peer network of miner nodes." The key thing is that since it's p2p, there's no owner of the network who can use it to perform a 51% attack.
P2Pool is not perfect, but it works and it shows that decentralized pools mining are possible. With some improvement, it (or something like it) could be a replacement for all centralized mining pools.