There are many future web stack features that I see as being vitally important to the long-term health of the web. These include extensible web projects such as web components and CSS Houdini, as well as the scripting capabilities in ES7 and beyond. These features give developers better tools, and more fine control and power.
But I feel that what’s more important to the immediate success of the web are features that provide parity with native mobile apps. I’ve written previously about the importance of service workers in providing this parity, but there are also a few new features breaking through that I’m equally excited about, as they provide access to previously unavailable hardware.