Microsoft just released into Targeted Release tenants the ability to add a mega menu navigation at the site collection level.
*If you are looking for a true global navigation solution, you will need to create a custom SharePoint Framework (SPFx) Extension
Mega Menu Navigation
How to Configure
To change the navigation to the mega menu, click on the site actions menu (gear icon) in the upper right corner of the screen and select "Change the Look" from the dropdown menu.