I would like to see server wide chat channels.
This means if you're typing in /trade or /advice, it's seen server wide, not just in the area you are in.
Then instead of having a general, trade, and advice channel for each area, there would just be a general chat channel for that area, example "The Harbor".
Using /say would send the message to "The Harbor" or current zone, and using general, trade, or advice would send the message to the server wide channel.
The only downside to this is people will say it will just be spammy / scrolling text, but let's face it, if you wanna sell an item via trade chat, it makes sense that everyone see it and not just the people in the marketplace or just in the harbor, and if you're asking a question in advice, you can get an answer faster if everyone can see it.
Then, if you're chatting about a quest in a certain area, etc., you can use the /say command to talk to just the people in your area.
For server wide general chat, I could see it getting a bit over loaded, but at the very least, for trade and advice, I would very much like this.
Also, it'd be nice if you could send a mail message with a COD amount, so that if you were using trade chat and someone far away wanted something, you could just drop it in the mail and specify the COD amount, and when they get the mail, detaching the item included would send you a message back with the payment attached. Of course, for this they would need to increase the amount of platinum you can send with one message, or either not include that limitation to COD requests.