Spartan Ops mode.
With this mode, players will engage in a co-op story with other Spartan trainees, not unlike the co-op of Portal 2. The difference with Halo 4 is that this story is episodic, with new missions being released each week. O'Connor said, "It's about the same amount of story as [a traditional] campaign stretched over weeks."
Besides giving players more bang for their buck, the Spartan Ops mode was created for another practical reason. Despite what a glance at the leaderboards seems to tell you, Halo players really don't play online. "Most people actually play campaign and don't play competitive. It's because the competitive landscape is really intimidating for people. So they prefer to play at their own pace," said O'Connor.
The Spartan Ops story will stretch out for several weeks, but not months, according to O'Connor. And beyond that? "Season one comes free with the game. If we did another season we would be building that from scratch. Maybe that will be DLC; maybe it goes in the next game. We are still tinkering with that."