There are some plugins that I use as much as, or even more than, XD's core tools, and it would be great if they were just as easy to access. Here are some UI changes that I think would make a huge difference:

FOLDERS
I'd really like to be able to sort my plugins into folders so that I can access related features at once. For instance, most of the time, I'd love to be able to move the preview and export plugins to a folder that I could leave closed while I'm designing, then open up for all-in-one-place access when I need them. (Related suggestion: https://adobexd.uservoice.com/forums/353007-adobe-xd-feature-requests/suggestions/39925795-favorite-plugins-list-in-plugins-panel)

CUSTOM ORDER
The ability to drag and drop the plugins into a custom order would allow users to put their most-used plugins at the top, and this feature would also facilitate moving plugins into folders.

PERSISTENT PANEL
There are some plugins that I want to have access to at all times, so it's a bit annoying that I can't see them at the same time as the Assets and Layers panels. Personally, I'd love to be able to open up one or more specific plugin folders in a mini panel, either undocked or locked at the top of the left-hand panel. Another option would be to allow multiple panels side-by-side (related suggestion: https://adobexd.uservoice.com/forums/353007-adobe-xd-feature-requests/suggestions/34812523-customize-panel-location-dockable).

CONDENSED VIEW
For users with huge plugin libraries, it would be great to be able to fit more plugins on the screen at once. An icon-only condensed view (show name on hover) would be a great solution--and one especially appropriate for single-click, single-action plugins, which are essentially just buttons.

A search field for installed plugins would fit in well with these changes. That suggestion is here: https://adobexd.uservoice.com/forums/353007-adobe-xd-feature-requests/suggestions/37944496-search-option-for-installed-plugins