The trailer pretty much confirms that the entity talking to Halsey is Chakas. "An alien AI full of human memories" Thats a pretty accurate description of Chakas as seen in Primordium, a Forerunner monitor with the durances/geas of numerous Ancient Humans stored within. Can't wait to see what he's got to say and do. "Jul 'Mdama's ultimate plan nears success." So the Librarian is coming back? "As Infinity's command crew makes a discovery that changes everything we thought we knew about the Forerunner Prometheans." Since they would already know that the Prometheans are composed humans then this discovery must be something different. Maybe it's linked to Halsey's "What gifts the Promethean lifeforms have to offer.." comment. Ancient human information, the Flood "cure", warning about the Precursers impending test?