My needs might change on a daily basis, so it'd be hard to know exactly the plugin needed at the moment it's needed, so I just try and read various sites, including this one, blog posts, Sketch forums, to try and discover new plugins, maybe download them right away, and see if I can remember to use them when the time comes. The ideal way, that no one has invented, would be you launch Sketch, a message pops up that says, would you like to see the plugins invented this week? You click yes, then it plays you a video compilation of all the plugins in action, at the end of each segment in the video, a button says, click here to install. Then you don't have to do any research on your own.

I use Sketch Toolbox to find and install, and try and keep up with Sketch news and forums, sometimes by just doing google searches to discover new ones.

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?