It kind of makes sense that it's her, because she's been the focal point of the entire series so far. Scott had another instance of major character development - he's been the other main focus of the series...I can't see him dying.
But I would expect that a 10 mile long bullet would destroy Earth, so I don't see Kitty dying because Earth of course will not be destroyed. So maybe someone sacrifices themselves to stop the bullet. Peter? Is he known to, say, jump long distances like Hulk? I don't think anyone else is strong enough to stop it (in other words, no Fastball Special) and there are no controls or anything to stop it (which rules out Beast or Kitty if she were to wake up).
I wouldn't consider Colossus a leaper. But we've already seen Pete use Kitty in a Fastball special. What's to stop him from using someone strong and able to survive outer space like Emma? Who just happens to need retribution lately.