I think the classic way to do this is to look at Reverse Depends, Reverse Imports and Reverse Suggests on CRAN. These are only CRAN packages but are, by definition, packages depending at some level on shiny / building upon Shiny. Obviously, many will not be extending Shiny in the way you are thinking, but there may be some gems in there, and all of them are likely using Shiny in some way that might be insightful.
Caution: the list is massive 
https://cran.r-project.org/web/packages/shiny/index.html