With the most recent release of Web App Builder for Developers v2.0 coming out yesterday, although great with all of its updates for 3D scenes and other widgets, is still lacking a core functionality of GIS - editing related tables.
I thought this shameless plea for Up-Votes on ArcGIS Ideas might show the Web App Builder/ArcGIS Online dev teams how much we care about editing related tables, in the hopes they might consider including this functionality in their next release (2.1?!) of Web App Builder. Of course, if any of you have created a custom widget, or are working on one to provide this functioality, etc., let us know here Web AppBuilder Custom Widgets)
EDIT 6/17/2016: Here is an updated link for the idea on the ArcGIS Ideas website: