Here, I am talking about the ability to take a scanned map image, say from https://davidrumsey.com and bring that image into ArcGIS Online to georeference it. Yes, there are workflows to go from the Desktop to ArcGIS Online, but this would allow many users to completely untether from desktop GIS.
There are currently a couple of web-based georeferencers available, including the aging MapWarper, implemented by NYPL here: http://maps.nypl.org/warper/, as well as Klokan Technology's Georeferencer, which can be experienced at https://www.davidrumsey.com/view/georeferencer. Klokan also provides a "pay to play" service that allows users to host a limited number of map images for georeferencing and provides GeoTiff download, WMS, WMTS and other services on the resulting data.
The Klokan georeferencer leverages the https://iiif.io/ Image Interoperability standard, which is quickly being adopted by many orgs in the Galleries, Libraries, Archives and Museums (GLAM) community for serving, sharing and manipulating imagery collections, including cartographic materials. DavidRumsey.com, as well as other venues like OldMapsOnline.org are using IIIF extensively, so a georeferencer that supports IIIF would instantly provide access to hundreds of thousands of historic cartographic resources, including the Library of Congress' Sanborn Maps Collection, which will soon begin moving into the public domain.
This is really the last bit of major functionality needed to make ArcGIS Online a true replacement for Desktop GIS applications, particularly for academic research in the Humanities.