Maybe something like LED flash for cameras for phones that don't have one? Screen extension or even Dual screen so it'll be that while you're watching a video on the main screen, the secondary screen can be used to look about on the menu or even with the bottom screen being used as a music player screen so you can just tap for the next song without having to go to the process.
Secondary battery as well comes to mind, if possible that is. There's a lot of opportunity all that needs to be done is to see who would do it. Picture it, if you're going to be on something like a 10 hour drive and you forgot to charge your phone which could soon die. You got not car charger and let's face it you probably wouldn't want to stop on the side of the road at some odd place just to charge your phone. If you could use it as a secondary battery you could have a few more hours of lifetime so then you can charge it when you get to your destination.
That's be pretty funky if you ask me.