Here's my Halo 5 prediction.
Cortana is not in Halo 5, but the game is about her.
Chief's story:
Chief goes rogue trying to hunt down some crazy forerunner tech to save cortana.
Chief finds out the only way to recover cortana is through the use of forerunner technology that bridges digital and organic life (but works, unlike the didact's prometheans). He is so focused on this journey he neglects helping humanity, which is again in peril. Playing as chief, we will get to explore new forerunner worlds, enemies and technology.
Locke's story:
Locke is part of an ONI anti-Spartan-going-rogue contingency program.
Not unlike jason bourne- when the multi-billion dollar super weapon goes rogue, the powers at be decide it must be eliminated to prevent the risk of it becoming an enemy.
Playing as locke, we will get to see first-hand some of the destruction of humanity that MC ignored.
I think the finale will be that Locke catches up to Chief at the moment he finally finds the tech to save cortana. Chief's perspective is that he needs cortana in order to be successful against whatever is threatening humanity (but he's also in it for selfish reasons).
While locke's perspective is that Chief is chasing a myth while humanity is getting destroyed right now.
"All hail the conquering hero. Let us remember him as our protector And not the one who gave us this [destruction]. As our savior, and not our betrayer... All hail the conquering hero- the one who was supposed to save us all! But now...I... must save us from you."
Edit: I don't normally make or read story predictions. Sorry if this has already been predicted or anything.