So we all know Cortana is on the verge of rampancy. Well Chief will want to get her fixed as soon as possible. What kind of race would have the technology to save her? The Forerunners. Now here is where the giant gray and orange sphere comes into play, which is called a Cryptum. A Cryptum can put a organism in suspended animation for thousands of years. But there is only one Cryptum in existence and that belongs to the Didact which there was also a hint of in the live action trailer where the red wave goes through the ship and the Didact seal is displayed on the screen. So that is saying that the Didact will somehow be involved. And the Cryptum may have storage for AI repair. So Master Chief will go to the Didact asking for him to save Cortana but he will deny him. And he and Chief will become more bitter toward’s one another, but the Didact will assist humanity because of the rogue Prometheans (Black) While the loyal Prometheans (White, as seen in the trailer) Have an all out war for Requim. The fight will be three ways against, Promethean vs. Covenant vs. Humans, all fighting.
Wow, that could work.
Personally, I don’t think the chief would simply ask the didact for the Cryptum.
I was referring to something that would fix Cortana, like tools in a lab (We know how massive that Cryptum is).
- Why would the Didact deny Chief help?
- Chief is a Spartan, I doubt he would hold a grudge like that.
- Rogue Prometheans?
Not saying it’s a bad theory, just some questions 
Rogue Prometheans could be a possibility. They could be leadered by the Forerunner AI corrupted by Gravemind.
Anything is possible at this point so yes maybe.
Ok
- The Didact could still hold resentment to humans after the war but views their cause as worthy and he needs help.
- Chief is very attached to Cortana, so he will do anything to get her fixed.
3.Like El Rey Ceurvo said it could be corrupted by a flood gravemind or a hacker or anything.