The new Bookmark Widget in the Web App Builder allows setting layer's visibility (check the initial Idea: https://community.esri.com/ideas/12630-layer-bookmarks-in-web-appbuilder-for-arcgis)
This widget can now be used not only for bookmarking spatial locations, but also to configure layers visibility, which is very useful if you have many layers in your web map.
However, options for setting up spatial extent are very limited. Current main limitations are:
- the extent of a bookmark can be set only by zooming/panning on a small web map preview (little control of the actual extent)
- it is difficult to use exactly the same spatial extent for more bookmarks (in case you only need to turn layers on/off)
- when you edit a previously created bookmark, the extent needs to be set again (it is not preserved / restored when you open existing bookmark).
Below is a list of suggested improvements:
-  add option (checkbox) for disabling "spatial extent" for bookmarks (as a result only the visibility of layers will be changed, while the extent of the map will stay the same - in this way the widget can be used for turning on/off groups of layers) - currently it is difficult (impossible?) to turn on/off groups of layers
-  add option for defining the spatial extent based on bounding box coordinates (alternatively this could be defined as a center lat/lon + zoom level)
-  add option for defining the spatial extent based on extent of a selected layer
- share/reuse bookmarks globally (within a Group or Organization)