Currently you can use an existing WebMap as the basemap for an Express Map. The documentation on the extent to which you can do this is not particularly consistent. Some places say only the layers in the layers in the Basemap group will be drawn. The tooltip in the dialog for selecting a WebMap as a basemap suggest all layers will be drawn, but without popups.
In practice, only the layers in the Basemap group are drawn. I suggest that the functionality implied by the tooltip in the Change basemap dialog be implemented. In other words, when selecting a published WebMap as the basemap for an Express Map, it should be rendered visually as it is in the Map Viewer, but with popups disabled. This would enable users to include reference layers that are in a feature services (e.g. watershed boundary, parcel, zoning areas, etc) that cannot be added to the Basemap group in the WebMap.
