Having seen all the cutscenes now, I'd say this game has a strong narrative. However, it's built almost entirely on breaking the big rule laid out in every previous game:
you don't show Forerunner.
Story-wise, it's a huge cop-out. Anyone could have written this.

The first trilogy had a well-connected, if not well-written, functional storyline: In Halo, we found a Halo and fought the Covenant/Flood. In Halo 2, the Covenant found Earth, and the Halo array was revealed to be a seven-ring system, ultimately armed and ready to fire. In Halo 3, we stopped that from happening.

But with this game, I can't see a direction to take,
other than the unimpressive option of having the new villain return with reinforcements (and we've seen that already with Halo 2),
which would tie the next two games into the supposed second trilogy they're making. I don't think it's accurate to call it a trilogy if it goes off in some wild other direction from here on, it would make Halo 4 more of an epilogue to the first trilogy, a stand-alone game.

Where do you guys think they could take the story from here that's not completely retarded? I'd suggest spoiler tags since some people may not have finished the campaign.