I'm late to the party but lets list some facts. First, the A.I. chip Chief is holding in the e3 trailer, is refereed to as a "Weapon Containment Device." Obviously this is going to be for current "evil" Cortana. The chip is also the next iteration of the CTN model according to the string of numbers following the model name. In Halo 4, Cortana split herself into multiple copies to save Chief and to stop the Diadact. When Chief and her are talking immediately after that, she mentions that "Most of me is down there,,," She is referring to the other copies of herself. Moving on to Halo 5, it is apparent that the more fragmented she becomes, the less stable she acts. This was also slightly hinted at in Halo 3. With all of this in consideration, lets speculate with included justifications. Cortana is more than likely still going through rampancy. Hence, her fragmented self. The chip will be used to contain her so that A) Halsey can fix her. B) Chief can destroy her. Or C) Another Cortana model will be in there, to slightly overwrite her. I like option C. This would eliminate it just being a completely new model. Instead, the memories, the knowledge, and the interpersonal relationship between her chief, wouldn't need rebuilt. While the rampant logic of herself, would be improved. I do not think this would undermine Halo 4 and 5. The reason I've came to this conclusion is that the conflict of the over arching story has always been to protect Cortana. She is refereed to as the savior of the human race. This was made apparent in Reach, when Halsey said, "When Reach falls... and it will, our annihilation is all but certain. Unless, we can glean from this artifact a defense against The Covenant." Another example would be in Halo 3. Sergeant Johnson, right before his death, he emphasized to "Never let her go." He did this to exclaim the importance of her survival. He knew she was a key part of humanity's survival. You didn't save Cortana from the flood for just any old reason. I would argue the death of Cortana, and now rampancy, was the ultimate retcon everyone keeps pointing out. Not that she comes back into the story. Objectively, this move would ultimately be best for the story. I say this because it sticks with the themes of Halo.