Lightning
I'm expecting something to happen to Gray Wing, honestly. Kate hinted at a heartbreaking death or something along those lines on her blog, and I wouldn't put it past her to kill him. If not him, then most likely cats like Star Flower, Jagged Peak, or another “major but not major enough cat to live”. Seems to be how they do it- they kill the ones we either adore (Turtle Tail) or apparently hate (Ferncloud died because of this).