An app that is a web browser, like say some implementation of the chromium browser, would allow internally linking to user created web apps that are hosted on the same server. Therefore these webapps would only be accessible to Nextcloud using internal addresses.
Examples would include setting up a FreshRSS server, or Shaarli instance that would only be accessible via a local web address and the internal browser would do that. Or making your own dashboard that is a simple html file accessible only from within whatever local network Nextcloud is running on.
I know Polar Bookshelf uses " Electron (which) is powered by the Chromium rendering engine and Node. js".
So maybe an electron app that runs inside nextcloud?
With a browser app that accesses internal links the possibility of users easily hosting their own apps is nearly infinite.