Most likely, one of the big focal points of the event will be "Sun Valley". For many months, there have been rumors about a big user interface overhaul to streamline the look & feel of the Windows operating system. It's believed Sun Valley will be rolled out via this year's Windows 10 Fall update.
Other than this, we may see some new features too, perhaps some functionality from the (already defunct) Windows 10X. Furthermore, there are whispers about a new and improved Windows Store that should offer a lot more freedom to developers.