Custom Libraries

Yes - this already happens. You have a few options:

• Libraries that are on espruino.com - for instance the graph library at http://www.espruino.com/graph get automatically pulled in
  
• Libraries in https://github.com/espruino/BangleApps/tree/master/modules get pulled in by the app loader and are included into the app's JavaScript
  
• Or other apps can provide functionality as a module as part of the app - like notifications. Apps can then request notifications are installed with "dependencies": {"notify":"type"},.
  
  

The nice bit about the third option is an app can request notifications, but you have a choice about which app you install that provides notifications (so you can for instance have ones that look a bit different)