This mega-menu is a page. You can add what you want on it : Blocks, layouts styles on blocks, ..
A page-type is installed with Anitya, let you create mega-menu in one click, the second one is to publish them!
Welcome to new Anitya I'm John,
The main navigation can display rich content under a page. This content is infinite possibilities because it is a stack of blocks created in the dashboard. It is possible for you to view the blocks and stack some blocks in a column.