Add a message system (/msg, /reply, /msgtoggle, /msgspy) including (if possible) a external placeholder for the msgtoggle status, i.e: (%chatformatter_mgstoggle% returns true/false)
for the format, it could be:

msgSender: To <recipient>: <message>
msgRecipient: From <sender>: <message>

Add mentions (@player), option to add sound when mention someone, and maybe a way to disable mention sounds (could be /mentiontoggle?)