When I observe page_block_html_topmenu_gethtml_before event, just like Magento\Catalog\ObserverAddCatalogToTopmenuItemsObserver does, and add some items to the top.menu, I can't order the items I add (whether it should be displayed before or after category listing, for example).
I think it should be possible, since the "top menu" must be one of the first and most modified element on any website.
Maybe by adding a special \Magento\Framework\DataObject attribute "order" which will be used inside or before Magento\Theme\Block\Html's _getHtml method, in a way to reorder the \Magento\Framework\Data\Tree\Node before rendering it ?
Or even clearer : adding a $_position property to the Node class, and a non-mandatory variable to the Node class constructor, which could be used in a similar way I expressed before ?
Simple, non-breaking, no performance issue, really useful. Right ?
I can try to make a pull request if you accept this issue.