Filter widget/functionality like that already built into the AGO Javascript map viewer: where in the latter you go Content->click the drop down arrow next to the layer->context menu opens with Filter option->Filter dialog opens and enables end user to create|execute|edit their own query on a series of expressions.
The WAB Query widget looks similar to the AGO Filter tool, but former doesn't allow end user to build their own query (as far as I can tell). Moreover, the WAB Query widget at beta 2 is so buggy and unusable, I'm not able to make sense of it.
At WAB beta 2, seems ESRI could just take the AGO Filter tool and add it to WAB (and even replace most of the Query widget code with AGO Filter code).