Hello!
I think it would be nice if you could implement XMPP (Jabber) S2S
(server to server). That is a protocol betwen servers, so a IM user on
one server (say yahoo.com) can IM a user on another server (say
Jabber.org).
For a list of servers that already have done it, see http://www.xmpp.net
For info on the protocol, see http://www.xmpp.org/
Sincerly
Sveinung