It would be very beneficial if users had the ability to customize symbology of layers in the Layer List Widget. Changing Line Width/Color/Polygon Fills would assist our users with color vision deficiency. It would also help reduce cost in printing, as Black and White prints would become much more feasible.
Ideally this preference would be set in the cache. The framework is already in place; Some settings are already cached, like extent and layer visibility.
This ability already exists in the Query Widget, which is wonderful. Maybe it could be extended to the Layer List widget. See Idea: https://geonet.esri.com/ideas/12640-allow-user-to-modify-symbology-of-query-widget-results
UI Builder Layout Ideas: