it might sounds stupid, but in my opinion before we concentrate on fancy functionalities like nice UI to adding grid, boxes, etc.. (which can already be achieved using plugins). We should address one very important flaw of the current editor, which is the fact that a user cannot easily add a line break, because as soon as you press Enter, the editor goes on a new paragraph adding a margin between the lines.

of course you can quite easily go in "text mode" and add a br tag manually, but most of the users don't know it, and it's quite an hassle if you have many of them within a page.

it is a feature that everyone needs, so I normally add a custom shortcode for BR, and explain the clients how to use it, but I'd love if there was a built in solution in the wp_editor, quick and easy to understand.