Here's what I think might happen, but first this. So we all remember noble six staying behind to operate MAC cannon so he could blow up that covenant cruiser so the Autumn could take off. Well I think chiefs death could be something like that but here what I think. Does anyone remember in reach when Halsey says that without cortana, humanity is already lost. And then in CE when Keyes said that if they got ahold of cortana that the covenant would learn about earth. Well i think that since cortana is gone. The covenant now has an advantage and may be able to launch an attack in earth again, cause i think it was only one attack that happened in new mombasa in halo 3: ODST. So i kinda think that the chief would die to basically somehow save the earth from being taken over by the covenant.