New Edge features
There's no doubt that the number one improvement that Edge browser is going to get is support for extensions, but Microsoft hasn't yet provided release date information for this particular feature.
But Windows 10 build 10551 comes with synchronization options that allow users to sync their content across devices. Edge browser will be offered as a universal app, so it works on PCs, tablets, and smartphones powered by Windows 10. The new sync features thus make it possible for those with more than one Windows 10 device to have their content wherever they go, which is a feature that's already available in other browsers currently on the market.
Another feature that's coming in Edge is the possibility of opening a new window with a middle click on the taskbar icon of the browser. Again, this option already exists in other browsers, so Microsoft might be following their example to improve its own app.
As far as the other Edge browser improvements are concerned, Microsoft is expected to roll them out sometime in the next couple of months, with many saying that extension support should be ready by November, not only for insiders but for RTM users too.