Well we could have:
A guild chat that only members of a guild can use on any planet.
A Private Message that can only be sent to friends on any planet.
A “global chat” that has a radius around the player and can only chat if they are on the same planet and could be up-gradable (pretty much what we all ready have).
Any way those are just my thoughts on the matter.
The idea about finding a more unique way around the problem is pretty good, How about we all discuss that and stop with the global chat for now, as I can see that it is still a bit “hot”.
Any way what about a Mailbox we can craft that can be used to write a message that then can be sent to another players mailbox.