For quick and intuitive navigation within a ArcGIS Online Web Application it would be very useful that the legend is clickable. When clicking on a legend item the extent changes to the extent wherein the item / layer is visible.
For example: in the viewer are presented a couple of GPX tracks. In the legend the GPX tracks are all visible under each other. When clicking on one GPX track in the legend, the viewer zooms to the extent where the clicked track is completely visible.
So, just a simple viewer with clickable items to zoom to
(like in the ArcGIS.com viewer, BUT without RIGHTclick)