idea - allow web viewer users to create their own labels on the fly. This could apply to WebApp Builder or really to any viewer with a layer list.
My thought is they simply have access to the label maker just like we do in the web map. It would be nice to even allow multiple fields to be labeled. But priority would be just supporting one. I have often had this thought while using my own property viewer in my city that I'd like to see Sales Price on each parcel listed, or perhaps Sales Price and Owner for example. I think power users would immediately and intuitively understand and greatly leverage this functionality. Just add it to the '...' button by each layer.