I have no idea. It's very similar to Halo: CE in that regard. I'm assuming Chief will be getting the new MJOLNIR Mark VII armor to replace the Mark VI at the very least. As for the story, I guess Spartan Ops is there to bridge the gap. And the books as well.
Ah, yes, the Negotiator: General Kenobi <sneaky beeping>
MC age doesn't matter, the Librarian spead up his evolution which is how he survived the beam that wasted everyone on that station. Regardless of his age, it seems he'll get stronger and stronger in this trilogy.
Also without the Didact(I think he's alive) between Halo 4 and 5 humans should continue to fight rogue Covenant factions and left over Promethean forces. I think MC will have a new AI.
for some reason i can see chief with an AI of Laskey, they put a lot of emphasis on him in the run up and in 4 for him to just be that guy you speak to every now and again in campaign, i think he's got a much bigger role to play. and lets face is Hasley made an AI out of her brain tissue, i'm sure she could do it again with someone elses tissue.
The Librarian and the Didact both mention or at least hint that the MC AND Cortana are special. We get that the MC is years in the making and being shaped to be genetically perfect, but they also mention that Cortana is a very unique advanced AI. I think there's going to be a thing where the Chief has a new AI, and is finding the "Mantle", if it is a physical object.
What I predict happening is this:
Chief is finding new tech on this forerunner world. New Villain (or the Didact if he's not dead) is released. Out of no where, the Librarian is also embedded on another planet, has a copy of Cortana, and bam they are reunited.
I know its a bit of a stretch, but when the Librarian grabbed Cortana, I figured it was more than just a briefing. They can fudge some lore and whatnot, but I do not think they would let Cortana go that easily. She's a staple of the series.
Also, I think MC will end up having new powers in the feature games, although this might make people bemoan the new direction. It would be cool though to see him use abilities like the Didact instead of AA, but that's deviating from this lore and reaching into ME.
There is no way the Didact is dead. He is a very important figure in the Halo universe and all that really happened was that he fell off the bridge at the end. Teleportation and levitation are both things demonstrated frequently by forerunner tech.
Considering that the MC survived a nuclear explosion at point blank range thanks to Cortana; it is very likely that most of the ship survived as well and it jumped to slipspace with her aboard. She is important to the librarian's plan as stated in the game, and the Didact found her interesting too.
I predict Halo 5 will deal with the return of the Didact and the rescue of Cortana who is being studied by him.
Life is an incessant series of problems... all difficult, with brutally limited choices - and a time limit.- Chapel the Evergreen