Magento – How to add Custom Menu item and submenu in navigation bar Magento 2

Late to the party here, and I think you've probably already figured it out by now, but you need to have categories UNDER the "Default Category". Then they should show.

I've also been looking into this for the past few days. The Magento 2 blank theme uses out of the box jquery ui for the menu. I was able to use CSS to stretch the drop down to full width but I couldn't get custom links working in this or multiple columns under one drop down. I think the only option is to write a module or plugin to reformat the html. Remember that if you write a plugin you can only use this to override public methods. I'm looking at overriding or writing something to run before/after the getHTML function in Magento\Theme\Block\Html\Topmenu

Another possibility may be to remove the menu.js jquery part and write something entirely custom for this but I think you still wouldn't be able do anything about custom links without a plugin.