Well, I just hatched a thought. Now, don't flame me for this- it's just my opinion!
Ok. So, how about the enemy get's a hold of Cortana and actually destroys her right in front of Chief. You see Chief slump while he's being held by guards, and then tense. The camera zooms in on his helmet as he raises it. Then he pretty much rages and there's an epic cutscene hand-to-hand fight (that the enemy gets owned in) between John and the enemies.
What do you guys think of that, and what do you want to see happen or want to see in Halo 4?