Esri recently introduced ArcGIS Online layout templates (beta) for use in the MapViewer (What's new in Printing from Map Viewer); however, these templates cannot be accessed by users through the print widget in apps such as Expreinece builder.
Adding the implementation of custom layout templates across printing services in ArcGIS Online and various WebApps would be a highly desired functionality as it would allow end-users who are interacting with a WebApp to print custom maps using a standardised template published by an organisation.