I've been using XD for a while now and have been very satisfied with the beta version, although there are 3 top features that would substantially improve the application:

1. Add the ability to pin certain objects to keep them from scrolling with the entire screen. For example, a navigation bar (header) or tab bar (footer) becomes static and does not scroll with the rest of the elements on the screen.

2. Add a few application preferences, i.e.: default font (would be a huge time saver to not have to change to the desired font every time new text elements are added).

3. The ability to drop default elements that would normally be available in Xcode. For example, nav bars, tab bars, buttons, segmented control, text field, slider, switches, scrolling text views, picker views, search bar, keyboard toggles, scroll view, etc.

Although there are other features like padding, default iOS elements, and constraints that would be helpful, these are the 3 features that would make the most impact on improving XD for me personally.