The Chinese have accomplished quite a bit with the service module left in space following the return of their latest lunar mission. I was wondering if something similar could be done using the 'trunk' section of a SpaceX Dragon cargo capsule. The DragonLab write-up says the solar arrays can supply up to 4KW, with a 1 year on-orbit lifetime. If it was left at the ISS it could become the basis for a good-sized free-flying satellite. 

Since this is such an obvious idea, I'm wondering if I have overlooked something …