It would be great to have better keyboard support as well. For example, using Esc to close editor menus, navigating with the arrow keys, pressing Enter to select options, etc.
A basic CSS editor would also be really useful, at least for things like creating or customizing a custom spinner/loading screen.
Another feature that could be really interesting is the ability to play/test the game directly inside the editor. I mean having something like a game window embedded in the editor, so you could test the game much faster and ideally make changes in real time without constantly switching between the editor and the game.
I think these features would make the workflow much faster and more convenient, especially when testing and tweaking UI elements.