Lightflame
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).
I was sure he was going to die in The First Battle, but since he survived, I don't think it's as likely he'll die now. Gray Wing is confirmed to father kits (with Slate, obviously), so I think they'll let him stick around to be with them. Plus, the main theme of the arc is loss and acceptance, so I think it would kind of screw with that if Gray Wing finds happiness only to die.
Jagged Peak would manage to be an equal gut punch without screwing with the themes of the arc.