We are a computer vision startup in Esri's emerging partner program -- our platform turns street photos from smartphones into 3D imagery (www.mapillary.com). Esri has encouraged us to look into building a custom widget -- we are new to WAB so any help or guidance is appreciated. We're currently working on an open source version of our viewer (https://github.com/mapillary/mapillary-js) and want to figure out how to build it into a widget. We hope the 3D landscapes will be useful telling stories and extracting data in apps for public works, parks, transportation, and more.
We see that rscheitlin made a Google Street View widget which looks like it works nicely. Do you have any advice for us?
Added your GitHub link to Web AppBuilder Developer Edition - Customization Resource List
look about half way down on the (long) list and you will find so links to custom widget dev. I know this area should be updated since there are probably more links from the forums and the help. In case these aren't in the list...
Look at widget development in the help doc Welcome—Web AppBuilder for ArcGIS (Developer Edition) | ArcGIS for Developers
for widget development
The api and interaction of your product and the Google StreetView are very similar. I would say that you or your developers could use my streetview widget as the base for your development with minimal effort.