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).
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.
The only way that would work and him still die is if there's a timeskip, honestly. I may have actually forgotten about the “he'll father kits” thing, since I'm so used to Pebble Heart, Sparrow Fur, and Owl Eyes being called his kits. I just hope that those three won't be forgotten in Path of Stars in favor of these new kits.
Eh, Jagged Peak wouldn't really hurt me as much. I don't care for him… although Quiet Rain's death was a huge punch to the gut and Sun Shadow dying would hurt bad well.
Basically the cats it can't be are:
Thunder
Clear Sky
Wind Runner
River Ripple
Tall Shadow
Gorse Fur
Moth Flight
Cloud Spots
Dappled Pelt
Pebble Heart
Owl Eyes
Unlikely, but still possible:
Lightning Tail
Gray Wing
Dust Muzzle (Kate hinted at him becoming Duststar)
Slate
Star Flower
Shattered Ice
Reed (he knows a lot about herbs, so maybe possible MC)
Fern
And free game:
Leaf
Nettle
Thorn
Acorn Fur
Birch
Alder
Jagged Peak
Holly
Mouse Ear
Mud Paws
Sparrow Fur
Eagle Feather
Storm Pelt
Dew Nose
Night
Dew
Slash
Snake
Blossom
Pink Eyes
Milkweed
Thistle
Clover
Minnow