The only PoS system in live and large scale active use from I have read so far (still got a lot of reading to do though so could be missing lots) is NEO, and from what I can gather the validator nodes need a stake of 1000 GAS which is currently a little over USD30K, so there isn't such a compelling case for a validator pool compared with the proposed 1500ETH with current market value of over USD1M for the Ethereum proposal.