All PoW schemes tend to concentrate miners in pools to have a more steady income for the miners, while the major PoS coins, such as NXT, also end up concentrating the stake because of leasing. But, does there exist any cryptocurrency that doesn't tend to centralization?
With P2Pool there's no central pool authority so it's as decentralized as it gets. The problem is that miners don't like to use it because it requires a copy of the entire blockchain. I hope this gets fixed with new bitcoind version that allows to prune and delete some history, and would therefore allow to run full nodes with less effort.
No. It is impossible. People always want to have more money/power than others.