Allow broadcasters and moderators to pin important messages in chat
This applies more to the mid to larger streamers but can be useful for the site as a whole I think.
If a person of importance is trying to communicate with the streamer then this would make it much easier for them. You may ask, why not just whisper? Well whispers aren't always suitable for those who have them disabled or for those who don't want to halt their stream to check them. The message may also be urgent so the streamer may not have time to check or may have their hands full.
This feature could also be used for mods to pin some sort of announcement in chat that viewers who just tuned in might have missed.
As for the pin feature, the broadcaster can set a maximum limit to how long these messages can stay pinned whilst moderators can pick the duration of time they want it to be pinned. If wanted, this pin option should also have the option to be disabled entirely for the viewer in their personal settings if they don't wish to see it obscuring chat. As well as this, the pinned message should have an 'x' next to it so it can be dismissed.