Thanks for asking. Here are some, a lot of which are simple:
1. SIMPLE: The ability to hide something in the tree view (even a top level item, like "Services & Plug-ins") that you don't want to see. Already possible with Zones.
2. SIMPLE: Use an icon other then the international standard "Play" image for linkable columns. Currently confusing.
3. SIMPLE: Add a real play button (perhaps in a column) to items in the top area, when that is shown as details list style. Currently double-clicking a details list item in the top doesn't play, it opens the file view below; you either have to right-click to play, or in thumbnail view you get a Play hover.
4. In the tree view, allow "manual" sorting, where we can just drag and drop something to a particular position and it stays there.
5. SIMPLE: Option to NOT auto-expand Playing Now in the tree. (Not really a change to the GUI, but it affects the look)
6. Add the ability to put command shortcuts on the toolbar. (Things like appear on the right-click recent command option, like On Disk-External).
7. Make Playing Now a separate pane in the tree (sort of like the action window) so that you can switch zones with one click without moving away from your current tree view. To switch to viewing the play doctor or current playing now, you could always double-click. Yes you can switch zones via menu, but it requires many clicks and mouse movements to navigate.
8. SIMPLE: The option to drill down all the way to files in the top pane, instead of opening the lower files pane on the last double-click. Essentially, the ability to use the right-hand side of standard view as a single unified pane.
9. In theater view, the ability to add playlist groups as a library item while still respecting playlist sequence order. (Matt fixed this years ago, and it's re-broken.) This is to enable Playlists to be inside a Music view, instead of as a top-level roller.
10. SIMPLE: In theater view, the subtitle selection OSD should be labeled the obvious "Forced Only" and "Off", instead of the current question inducing "Off" (when it's not off) and "Null".
There would need to be a whole thread on improvements to the visual look/operation of Theater View and its OSD.
(edited to add #10)