It would be great if the source data for map layers in AGOL could be changed in the same way as ArcMap. For example, we currently have a parcel layer in many of our web maps. We had to create a new parcel hosted feature service and get rid of the old one. We had to then go into each map, add the new parcel data, and change all of the symbology and labeling to match the old parcel layer. This could've been avoided if there was some way to change which hosted feature service was being used by the map layer, similar to how in ArcMap you can right click on a layer and change the source data.
This idea has been merged with Editing the URL path of a Web Map in ArcGIS Online.