A much nicer, user friendlier, preferrably React-based SiteTree editor. Dragging&dropping, ability to easily nest multiple levels, immediate visual feedback if a drag-move is not permitted. Big one, not being able to drag a page that is "locked". Handled with callbacks/Graphql and in React it would sport way better UX, handling edge cases & permissions to disable/deactivate blocks and prevent certain actions etc. Something like http://webcloud.se/react-sortable/ or building upon the concept of the WordPress menu editor.