During the events of Halo 4's Spartan Ops, many things happened that changed the plot of the game.
One of these things is the Janus Key. It was given to Doctor Halsey who was ordered to be assasinated by ONI. The Janus Key as said by the Librarian, shows the real time location of every piece of Forerunner technology in the galaxy. The key was ment to propell humanity to a technological tier 0f 1. On Halo Waypoint it stated the future of the Halo universe depends on the key. This means the key is the main Forerunner artifact that will be focused around the series for now. Dr.Halsey betrayed the UNSC in Halo 4: Spartan Ops and, Thomas Lasky, the Captian of Infinity was ordered to assasinate the rouge fugitive. Thomas Lasky did not want to uphold the task however. In the game he states that he is uncomfortable killing a civillian. I think it has more to do with than just that. In Halo 4's campaign, Cortana is in Rampancy, and the only way to fix the Rampancy is Halsey. In Halo 4's ending the, Epilouge. There is data seen coming from the ground. This data may be fragmented parts of Cortana. THe UNSC may have recovered this data. Thomas Lasky may have known that Dr. Halsey could restore Cortana for his emotionaly unstable friend, John-117. He may have not wanted her killed because she was the only one who could restore Cortana for the Spartan.
With this in mind Cortana could come back in future Halo games including Halo 5. I know Dr.Halsey is with the new Covenant, but who is she more partial to, A group of aliens she barely even trust, or her Spartans who she cared more for than all of Humanity. This was shown when she tried to save the rest of the Spartans from being killed even at the cost of the rest of Humanity. Thomas Lasky is very friendly with the Masterchief. Even though he is higher rank than him, he sees chief higher than himself. This was demonstrated when Chief called Lasky sir. This made Lasky uncomfortable and told him "At ease chief. It feels kind of odd for you to call me sir." Lasky could restore the Spartan's AI though. This is just my opinion however. Leave a comment below and say what you think.