Back in 2013, the Wi-Fi Alliance created a certification program for 802.11ac devices to ensure interoperability. That program is now expanding with a new set of features that the Alliance is calling "wave 2." Rather than being a new Wi-Fi standard, the new features supplement the existing 802.11ac standard.
The focus of the new features appears to be on improving the performance of Wi-Fi networks when multiple users are using the same network, and when multiple networks are occupying the same airspace. Multiple-input multiple-output, or "MIMO" support, was introduced originally with 802.11n. That feature is being expanded to multi-user MIMO, or MU-MIMO. Suitably-equipped access points will be able to serve multiple devices in MIMO mode simultaneously.
Beyond that, the majority of the updates seem to simply be specification upgrades to the certification program. Devices with 4x4 MIMO support are officially supported, as well as 160MHz channels—potentially doubling throughput. The Wi-Fi alliance is increasing the number of available 5Ghz channels, too, which it says makes more efficient use of available spectrum.
Unfortunately, the 802.11ac "wave 2" features do require hardware support, and as a result it's unlikely the new hotness will come to a router near you by way of a firmware update. Wave 2-capable devices are already on the market, though, like this Netgear R7800.