For SMP you should only be able to hear someone chat if you are near them, within "NEAR" fog of vision.
Also there should be a shout option, so you can chat to everyone on the map.
This has nothing to do with the main client, but servers should be able to network, such that they honour each others ops players but exist on multiple worlds. Currently some custom servers support multiple worlds, which sort of breaks the player base up. If you gave every server a world, and let people move amongst them.
Obviously coalition would be entirely up to server owners.