The ability to view Feature Layer related records via ArcGIS Online (AGOL) sounds so promising! I created a web map that displays a fGDB derived Feature Layer, and the results were exciting. I was able to display Feature Layer related records by clicking the "Show Related Records" hyperlink inside of the Feature Layer pop-up. Disappointment soon followed when adding that web map to an AGOL web app template; the pop-up window did not include the link "Show Related Records" as it had in the map viewer environment.
How important is it for you to be able to easily display Feature Layer related records within your organizations' web apps?