I've never really believed that Cortana died. Her "death" seemed to have happened while she was in a forerunner computer system that was designed to digitize intelligent beings and recreate them as new psychical beings. And even though the ship was destroyed he saw in SpOps that those people who were digitized still made it back to requiem to be made into warriors.
Halo 5 will will not only see Cortana again, but we may very likely see her as a physical entity. Mark my words.