My very first reaction to XD. Allow dragging left and right to change numbers in the in the property inspector instead of only being able to type numbers in there manually. My second quick reaction is that while the pale gray is fine and obviously utilitarian, it's also pretty boring. It takes minimalism to a new level. I would think that designers might like a little more color, contrast, and even larger-sized icons as a start, for the UI of a software for designing UI's.

This last comment, regarding the prototyping, this may not be the highest priority, but since you're building this from scratch to be a modern tool, I'd love to see more visual depictions of your stock transitions, for example, placing an animated gif of a blue square sliding left in the pop-up box that appears when dragging a noodle from one artboard to another, rather than just having all text with drop down menus. Maybe you have arrows to click through a gallery of animated gifs depicting transitions rather than drop-down menus. It's nice that there are point and click options, but it's still text. In the same way icons have replaced text for tools for wireframing, why not tiny animations to show options for interactions and transitions in prototyping?