What if when they kissed in that miserable cell, it didn't stop? What if he didn't pull back and run as fast as he could from her. What if instead of running, the Doctor gave in to the inevitable? Some spoilers (particularly from Day of the Moon) but i dont think i touched on anything too major.

River leaves the Doctor a little gift from his future and her past, and all that lovey-dovey romantic stuff kind of goes out the window. Smuttiness ensues.