The Ethereum Metropolis hardfork will be happening this month with a tentative date set at September 22.
According to Ethereum developers the first part of the Metropolis hardfork is in its final stages with ballpark block number at block 4.3 million on Sept. 22nd if things on schedule and at block 4.4 million on October 27 in worst case scenario. The developers added they are aiming for sometime late September.
The main aspects of the upgrade is a reduction in ethereum issuance to miners through block rewards from the current 5 eth to 3 eth while delaying the increase in mining difficulty which was placed there for the Proof of Stake (PoS) upgrade. PoS is delayed, so to keep issuance around the same level as it would have been had difficulty not been delayed, they are suggesting a reduction.
The Metropolis Byzantium hardfork is to then be followed by Metropolis Constantinople. There won’t be an Istanbul as there are only two parts, with the latter part probably to be implemented sometime next year.
The Proof of Stake hardfork might follow thereafter, with 2018 now seeming optimistic as there hasn’t been much update on its development while there have been some suggestions sharding is moving forward.
A number of new developers have apparently been added to the ethereum team, so matters might more forward more quickly, but with each difficulty delay the dangers of complacency increase.
Second Layer protocols like Raiden and Plasma may assist, but ethereum might also find itself in a situation where fees considerably increase and transactions are significantly delayed. That may make some smart contract based daaps inoperational, so further delays of Proof of Stake may not easily be afforded in the coming months.