This post is for fun, doesn’t have any facts but ideas of what could happen in the new trilogy. Please, if you know what’s going to happen don’t post it, if you have any creative idea or theory, be my guest and post it.
Well my theory is about the following:
Since Cortana is reaching a dying time, she will die eventually either on Halo 4 or 5. However, Chief needs her both emotionally and objectively. Apparently she is the only AI who can access somewhere and prevent a catastrophic event for humanity (can be giving power to the new threat which threatens Earth, or a huge explosion or a Halo size event which would most likely wipe out all sorts of life in a huge radius).
Since Chief now needs her to save humanity once again, he has to look all over the Shield World or other places to a solution of how to bring her back through Forerunner technology or human technology. Until he finds it on either Halo 5 or 6 and ends up saving the day.
Here comes the tricky part, if lets say that she dies at Halo 4 and the new threat starts to get power by something she could only stop (getting forerunner technology, or some sort of resource that enchances their power), the new threat would get more powerful in Halo 5, and by the time Halo 6 comes out, they would be like the Flood were in Halo 3 (growing and trying to become the executioner of both: Humans, and Covenant), as well as big in numbers and growing in power.
(there might be some facts wrong from pasts games)
Thoughts or ideas?