The upcoming Metropolis hard fork has been known from the start with Ethereum. Vitalik Buterin designed Ethereum to "Blow up" after 2 and 1/2 years to force the community to come together to make the changes necessary to upgrade the platform rather than having initiate any changes at all.
At a key note in late January Vitalik discussed this upcoming hard fork and mentioned the main reasons were to implement as many new features and upgrades as time would allow. With so many upgrades planned for Ethereum they may not even have time to include everything in the upcoming hard fork, having to wait until the following hard fork to implement the rest.
The most predominant and possibly largest change, will be privacy. Specifically the implementation of something called zk-SNARK. This is kind of a "super protocol" said Vitalik, allowing the a sender and receiver to interact without exposing the data. Vitalik explains this will allow for "proving things about encrypted data without revealing anything about the data, except the claim that you're proving".
This will be accomplished through Casper, which is a method of proof of stake rather than the current system of proof of work. The main benefit here, is proof of stock allows for better protection again wealthy attackers with expensive and new equipment being able to relentlessly attack to no end.
The exciting thing about Casper, is that it wont solely be confined to cryptocurrencies but to all applications across the Etherium platform. The most anticipated of these are privacy platforms, such as voting. The data can be proved without revealing the information, and cannot be altered.
Vitalik Buterin in Singapore keynote on Metropolis Jan 28 2017.