soo, heres another one. Someone enters an arena, and right clicks a challenge sign. and it announces to the server "PLAYER is waiting for challengers in the PvP Arena!" or "PLAYER wants to spleef at the Spleef Arena!" maybe configurable so that they can only announce like once every minute so that it doesnt spam the server.
I think this could be done with a bit of redstone and scripted signs in CraftBook... Though I am really not sure...