Ok. I've never done this before, so bare with me.
So, you know about Cortana, right? If you didn't know about how AI's were made, then you're about to.
If you read the Halo Fall of Reach book, the first book of the Halo books series I may add, then you learn about half way into it how AI's are made. Dr. Halsey explains this when Cortana comes into the story and the Dr. asks which Spartan she wanted to be paired up with. Of course, she chose Master Chief, but has anyone wondered why? It's because Cortana was made from Dr. Halsey's brain.
In the book, when Cortana told Dr. Halsey that she wanted to be paired up with Master Chief, Dr. Halsey wasn't surprised because Master Chief was one of her favorite Spartans. (The book made it sound like she was talking about a boyfriend/fav son).
It explained that an AI is created from the brain of a human. But in order for the process to work right, the human that is having their brain copied as an AI, has to be either dead or dying. Only because the big drawback of the process of making an AI, is that the person dies. The operation is so violent per say/so damaging to the brain, that it kills the person. Then how, you may ask, is Dr. Halsey in the other Halo games? There's a simple explanation for that.
It's because she clones her brain, which took about a million times over, and used the cloned brains to make Cortana. So in a sense, Cotana and Dr. Halsey are the same person. One is just younger then the other.
I'm saying this at this time because I had just seen a youtube video on this theory just before I started writing this, about how Cortana may be brought back in Halo 5.
Anyway, if they are bringing Cortana back, then it definitely won't be as a human, like the video said. That's my opinion though, (I can't stress that enough). The only reason I say this is because if she were to come back as a human, then it would have something to do with Dr. Halsey.
I thought at one point that Halsey had died somewhere in the Halo story line, but i realized after watching the Walkthroughs that it was a different doctor. My mistake.
Anyway, Cortana would have to come back, because you cant have Halo without an AI. Unless they are going to bring in a new AI. I think they would because of one of the other Halo books I read had on the cover had what looked like an evil version of the Librarian that looked like it worked for the Prometheans. Unless that was the Didact on the cover. Anyway, if I have any other theories, I will let you guys know. See ya'!
P.S. this is the link to the video I was talking about before. www.youtube.com/watch?v=cJL9AXislVY