Not sure how hard this would be to impliment since you have several different client apps, but I think adding in or expanding a flexibale messaging API would be very useful.
Imagine being able to send out messages as text on screen or even audio to any playing devices listed in the Plex server. Tell the kids or wife it's dinner time. You could also be moving toward better integration into smart home appliances as XBMC has where a appliance could message you through the home media center. So say you have a driveway motion sensor and your in your basement watching a movie on your 1000 watt surround sound syste. You could have a popup message or audio alert show up on the screen.
I think this is definitely something that all media centers will offer in the future, just like TVs often have a caller ID feature. I believe XBMC and WMS both have this capability. XBMC can even send images from your home survalance based on smart home events.
