The visual builder is great for general design, but is missing a couple of features I think would be great for transactional emails. First would be a way to create a "component" with multiple basic design elements. A component would be a group of titles, text, image, or other elements. The component can then be dropped multiple times and have its own variable references. The second is table support. Pulling from variables, it would allow for the building of tables on a referenced variable (that contains array of variables.)

These are things that can be done via handlebars and HTML templates, but being able to expose this via the visual builder allows for non-devs to manage templates.