I would like to see an addition to the mapviewer in ArcGIS Online. Now, you can see a scalebar, which changes if you zoom in or out.
I would like to see a text scale, i.e. 1:25.000, which you can edit to zoom in or out at your level you like. Maybe in addition to this idea, an checkbox to lock this scale. It's easier to specify on which scale you'd like to operate and if you have services which are only visible at 1:5.000, you can enter this and the lock the scale.