As it’s a network-based server, Universal Media Server doesn’t have a lot of scope for adding utilities. However, you can add streaming video feeds. In particular, the official wiki explains how to add YouTube channels, which present themselves via the interface of the DLNA client you are using.
Kodi
has an extensive add-ons ecosystem consisting of official and third-party modules, and these are installed from within the configuration user interface. In fact, the selection is so extensive that it borders on overwhelming. The add-ons consist of smaller utilities such as a weather app and game changers such as support for emulators and other ways to play games. Adding a web browser can be useful,too. It can be fiddly to locate a plugin and configure it, but nearly anything is possible once it’s working.
Plex
isn’t just a conduit to your own private media stash, it’s also a significant source of ad-supported films and TV shows, and your own media is mixed in with all of this when using the official app. If you’re already watching Plex, it might push you in favour of using the Plex server for your own media. Plex has an add-on system that is largely made up of utilities to improve interoperability with other services.