I don't see windows phones getting battery cases anytime soon. They just aren't popular enough for the Mophies of the world to make a case for a phone that has a body that wont even be used on its successor. My $.02
I been thinking about the camera grip too. But I want to find one in a store and try it out - the couple of centimetres extra length will, I reckon, mean I can't carry it in my pocket anymore - it may be a no-go.