In this case I created a custom entity called 'submenu'. In my custom module a used HOOK_LINK_ALTER to make the submenu (which contains a textfield) visible on hover. I used the code below to render my custom entity content:
When using lazy loading of images, I encountered an important issue. Social sharing engines like Facebook can't read placeholder images. In this drupal 8 tutorial we'll fix this issue by creating a small custom module handling image tokens.