Hmm, I think you'll see the following in Halo 5:
1.) A longer prologue that delves deeper into Johns training with Senior Cheif Mendez and his relationship with Halsey and the other SPARTAN-II's.
2.) Some kind of rift will occur that alienates John from the SPARTAN-IV's revolving Dr. Halsey. John possibly helps her escape?
3.) Many of the missions will be lone wolf with Master Chief fighting Covenant, The Didact or some new kind of promethean warriors ( possibly), and the SPARTAN-IV's
4.) Master Chief will begin some form of physical/mental ascendance due to the DNA upgrade from the librarian. He will also have a more intimate role with the librarian and Halsey. Protecting them? Working with them? Helping them reach some sort of goal?
5.) The Terminals of Halo 5 will delve deeper into the pre-history of humans.
6.) He wil recieve another Cortana AI, but she will have a different personality...it just wont be the same and Chief will react to this. The two cortana's will confront eachother in halo 6
7.) At the end of the game, master chief will suffer an almost fatal defeat against something or someone. Before this, he will briefly have contact with the remaining SPARTAN-II's and Senior Chief Mendez. For some reason, I foresee Master Chief completing the final mission without any armor. His armor having been destroyed or removed.
8.) I believe he will be close to death by the end of Halo 5.
9.) Epilogue of Halo 5 will bring an optimistic message to all Halo players and will show SPARTAN-II's and Senior Chief Mendez helping Master Chief recover, while Dr. Halsey and the LIbrarian make the finishing touches on a revolutionary armor system to match Master Chiefs progressing advances in his DNA that will help him make the ultimate come back in Halo 6.
10.) Random: Legendary ending of Halo 6 will be an elderly man and woman sitting in a futuristic living room. There will be a knock on the door. They go to answer. " Who are you?" you hear the old man say. "Your son." Says a wavering John. That would be a great way to end the Halo story.