One of the best ideas I have heard recently is that there needs to be a registry for proprietary GeoPackage extensions. While OGC has a number of registered extensions as part of the encoding standard, is it only natural that there are going to be proprietary ones out there. What we don't have today is a way for developers to see what exists as prior art. We then run the risk of developers reinventing the wheel and the longer this persists, the harder it is to deal with. The last thing we want is multiple competing extensions out there and no interoperability between them.
I believe an extensions registry will address this problem effectively. I will do the legwork for creating it. In the short term it is going to be pretty simple - just a web page off of geopackage.org with a bunch of hyperlinks. I will use the Leaflet Plugins page as inspiration; hopefully one day we will end up more like QGIS Plugins but it won't happen overnight.
Update: this is now live at http://www.geopackage.org/extensions.html.
To be added to the registry, I will need from the developer a completed Extension Template . This template includes all of the information developers will need to understand what the extension does and how it works. I will post the template to geopackage.org (or more likely, link to it) and help publicize it. If the extension suits other people's needs then great! If a number of implementers start using a particular extension, let the GeoPackage SWG know and we will discuss whether it is ready to be adopted by OGC.
 The raw AsciiDoc of the template is on GitHub. If filling out a template is going to be a major problem, let me know and I'll see what can be done to help you out.