Is it possible to have a drop down list for items already in a drop down list in the top bar navigation? Say you click on the first drop down, and it gives you 5 options. Each of those options then have another drop down list with additional options. I'm using SharePoint 2016.
If you want to extend the global navigation with a drop down menu than you have to modify the menu control in the master page with help of SharePoint Designer. Change the property for MaximumDynamicDisplayLevels from 1 to 2 to get a drop down of two levels. You need to find SharePoint:AspMenu and the above tag will be inside this control.
If you want to customize the global navigation using some CSS & JS than you can follow steps mentioned in below article.