As you already know you can manage Office 365 Administration Role assignment from the Active User page.
Well, if you are already using the new administration portal (in preview) 
you will be able to see a new blade in the left navigation pane called Roles which includes only one subsection Roles (maybe there are planning to add additional capabilities)
From this Roles\Roles blade you will have the complete list of available administration role and then get details about the selected role, including managing assignment
You also have the ability to download a CSV file listing all administration role with assignment (this list does not include role with no assignment)