This idea has failed to gain traction, and we will close it. Please re-submit if you'd like it to be reconsidered by the the ArcGIS Dashboards community.
I'd like to see this implemented, too. Now that we have Exclusive Visibility (radio buttons) for grouped layers, users should be able to easily toggle between layers in a dashboard.
There is a map legend dashboard element, but without a similar layer list element, it takes 3 clicks for users to switch layers:
Turn on the layer list (because it cannot be opened by default)
Expand the layer group
Select a layer
Dashboards are intended to be used by the public, and the steps to choose a layer are not clear to users who don't regularly work with the ArcGIS Online Map Viewer.