With the upcoming integration of PowerApps and SharePoint Online (SPO), user experience when editing metadata in lists and document libraries is also being improved (Note: PowerApps integration in SPO is currently available in Office 365 tenants where First Release is enabled) so users can be very productive when working with Metadata:
Browse a document library in a SPO site and just open the document details panel. From this panel you will be able to edit all the document properties using the “Edit all” link:
Then, a new metadata panel is shown where the user can change document metadata without leaving the initial screen:
Of course, if you have several content types in your document librart, you will be also able to change the document type to be used with the document library.