From what I have read Portal and AGO only support On Demand mode for feature layers. It would be nice if we could load layers in snapshot mode so that we could reduce the amount of queries we send back to our server. Furthermore, it would be nice if the supporting widgets worked off data stored in the DOM rather than always sending queries back to the server. I understand snapshot mode works best when there are relatively few features to draw but most maps seem to meet that requirement. This would also lower the load on service requests in ArcGIS Server.
Selection mode would be great as well. We have found selection mode to work nicely when we are working with very large temporal datasets. We have successfully loaded layers in Portal maps in selection mode but we are forced to add them in programmatically.