Just in case there's someone out there who hasn't played through Halo 4's Campaign yet or finished Spartan Ops, spoilers are ahead.
Well, Halo 4's Campaign ended fairly conclusively. The Chief destroyed the Composer and apparently killed Didact, and Cortana is presumed dead as well after her heroic sacrifice, and the Chief made back to Earth, being welcomed as a hero. Not much of a sequel hook there. Then Spartan Ops comes along and finally does some building towards Halo 5. At the conclusion of the second Requiem campaign, the planet was eventually destroyed, and presumably the entire Promethean army with it. Jul Mdama survived, and Halsey has ostensibly aligned herself with him, claiming she desires vengeance against the UNSC. But despite that minor sequel hook, the utter destruction of Requiem doesn't leave a lot left for Halo 5, and we're in all likelihood losing a major faction that was introduced and seemingly wiped out in the course of one game.
So, where do we go from here? Are the Didact and Cortana still alive? If the Didact is still alive, then how? Requiem was destroyed, so if he returns that means he ended up somewhere else. But even then, he's lost his entire Promethean army. So what does he do now? If he's dead, well, I guess he's just a throwaway villain, and a rather shallow one at that if I must say so myself. If Cortana is still alive, then what happened to her, and how will the Chief get her back? If she's dead, then it will leave a huge gap, as the Chief has lost his closest friend and companion, and not having her ever-present voice in his head will be quite... "disconcerting" I guess would be the word. And what of Halsey and Mdama? Is she just manipulating Mdama with her stated desire for revenge? What are her goals, exactly? Will she and the Chief ever meet up, with her becoming the new yet familiar voice in the John's head? Finally, what of the various other factions on Earth and Sanghelios? Are the Servants of Abiding Truth still a threat? Will the Arbiter have a significant role? Will ONI become one of the new primary antagonists, perhaps resulting in the Chief fighting the first human enemies in the series?
While I felt Halo 4's writing to be overall weak by Halo standards, I am nevertheless interested in seeing where the story goes in Halo 5. For the first time in a while, we have absolutely no idea what direction the story goes for the next game, even in the most general aspects.