Well, we got rid of global chat, but with the state of /s chat, it's almost just as bad. Even if some sort of "yell" command were introduced, I still don't think it should reach as far as regular chat does now.
The current chat pretty much forces most chatting to be done via tribe or PMs; if it's anything RP or PVP related.
I definitely think local should literally be local only. Think they could introduce a (optional) trade channel that spans a couple zone radius or maybe each zone could have a message board/totem. Players could leave messages like 'new eden is looking for sand, will trade obsidian. Zone whatever'