I can see NOTICE being handy for mods, but other than that I think private messages would be a bad idea.
I’d like to see a list of all the CSS tags/id’s for styling the web chat, so that developers don’t have to waste hours (not to say days) trying to figure them out.
I find the chat API very lacking, not to say non existing. All there is is /emoticons and /badges. I’d like to see a WebSocket API for instance.
IMO a must have is the ability to pass an existing token to either the chat <iframe> implementation or the chat popout:
http://www.twitch.tv/:channel/chat?popout=true&token=:oauth-token
The same goes for the player by the way.