Hmm ok. Well some advanced features I'd love for it to have in the library are:

:arrow: an Account Management System
:arrow: Chat Rooms with logging and wisper capabilities
:arrow: some kind of Host/Join Game System with Password, player limit suppot and other customizable options while keeping it 'open' for customization to a specific game's required options.
:arrow: Ping/Lag functions to detect a player/server's network strength.

Thats all I can think of right now...