I thought this was a plot thread.. meaning a thread to discus what you think the campaign could be about.
I think that the threat will still be the didact, in some shape or form. Maybe the flood returns and takes over his body, but since he is powerful he has half control? Master Chief discovers this when he is back on the infinity. As he is walking through the infinity the covenant attack and as they kill one of the key characters Master Chief over hears them in the radio saying "We have come for the parasite". Chief is confused and races towards the research and development lab and demands to know where and what the parasite is. They say that the parasite is locked in a secret facility in sector (insert number here) and that they can launch an escape pod for him but that is about it. So chief then has to find the escape pod, kill any covenant on the ship, and make his way towards the sector they told him the parasite was in. As he reaches the pod, he is picked up from behind and thrown onto the ground. As he turns around and pulls out his pistol he discovers it is arbiter, and arbiter discovers it is Chief. Arbiter demands to know where the parasite is, and Chief asks why and what it is. Arbiter tells him that it is the last remaining flood specimen in existence. If not destroyed it will jump at any chance of taking over a host. Chief knows he needs Cortana, and his search for her continues (the game starts with him looking at graphs and charts etc. of where she could have wound up.) He begins to search the three areas that he traced, and finds evidence of the remains of an AI. He winds up finding Cortana in 2 pieces, her Rampant half and her Sane half, but he needs both to repair her. So he takes Cortana and he asks her about any info regarding the parasite, and Cortana informs him that the UNSC was planning to test on the parasite to see if they could gain power of it and use it to produce a cure to soldiers with severed limbs etc. Chief then races his way back and races to the sector where he was told the parasite was being held and he discovers that it has already manifested and taken over everyone at the facility. At that time, the flood are even stronger as when they are shot they can regrow their severed limbs. Chief fights his way to the research facility and inserts Cortana into the database and she discovers what tests they were performing yada yada. So chief finds that though the parasite is still dangerous, it does have a mind of its own... it knows what it is doing now. It has already left the station in search for what it knows as the didact............ didacts body becomes they new gravemind.....