A release date for GDR3 has yet to be announced, but thanks to information from WPDang, they have learnt about some features from the GDR3 update that we might be able to expect. We had previously reported that notification could be a feature in GDR3, but apparently the feature is still being tested so there is no confirmation if it will be included in the final release.
There is also talk about individual volume control between apps. What this means is that users will be able to select volume levels depending on what they are doing on their phone, such as volume levels in general, volume levels for their music player, for their games, and etc. There is also talk about a Quick Settings menu that will be making its debut in GDR3 as well. As it stands the update is currently being tested internally at Microsoft and hopefully we will learn more about it in due time.