First off I just want to congratulate 343i, I think you guys have achieved a massive accomplishment with Halo 5: Guardians and should be commended, the amount of love and dedication really shows in the final product we fans have received, same goes for Halo 4. I honestly couldn’t be happier to have the franchise I love the most in gaming in your hands <3.
*This is gonna be a bit of a wall of text so ill break it down a bit
Hunt the Truth, the build up to Halo 5 and the groundwork laid for Halo 5’s story:
Hunt the truth was incredible. Its the best marketing campaign I think I have ever witnessed, please please please bring it or something similar back for the lead up of halo 6. I don’t think I have ever in my life been so excited to “listen” to something. Roughly speaking it created a story revolving around ONI being up to no good and toying with the idea of making master chief look like a bad guy… the lengths they go to achieve their goals are pretty horrible but it creates a really interesting story. We all know Chief is not the bad buy but why was he there when the guardian attacked? If ONI was capable of what they did during Hunt the Truth, what could the be capable of with the power of the Guardians? etc. Based on listening to Hunt the Truth my anticipation of what kind of story I could expect from Halo 5 story is the following:
I guessed that ONI knew about ortana being alive, Knew about the guardians and the power they possessed. They also were worried about an uprising with MDamas covenant and were weary of the fragile peace between humanity and the covenant. I figured they would want to use cortana to control the guardians for the purpose of Humanity using them against the covenant, both MDama and Arbiter, as that is the only way they could be 100% certain that Humanity would never have to face another war with the covenant. The problem is the chief would never A) trust the prometheans after the events of halo 4 especially with Cortana and B) allow humanity to attack Arbiters forces. So if ONI creates a narrative to make humanity turn on the chief it could also persuade the UNSC to believe he went rogue and assemble a task force of other spartans to hunt him down and prevent him from disrupting the plan (enter: Osiris).
so…Chief finds out what ONI is trying to do and with the help of Blue team is tries stop cortana, who is still rampant and is being manipulated by ONI, from assimilating with the prometheans to destroy the covenant. Locke and friends are at the same time thinking that chief is the bad guy and is causing the guardian attacks on human colonies. They are trying to stop him from destroying the world with cortana. Admittedly I assumed the majority of play would be with the chief while locke kind of shows up to foil the plans and also appears in cut scenes. Since halo 5 is not the end I assumed Osiris would find out what is really happening and join blue team to try and stop ONI but by then it will be to late, they would have control over the Guardians and set foot to attack senghelios first as they know they would be allies to the chief. so the game ends with saving Arbiter and escaping senghelios… epic warthog escape from exploding planet anyone?..
this would be setting up halo 6 to be an EPIC Blue Team + Osiris + swords of senghelios vs ONI / UNSC / All of humanity who thinks chief went rogue + the Prometheans… ONI could even use the prometheans to manipulate MDama, who views them as gods, or the brutes (as we know they are coming back in halo wars 2) to help them (even though they want to eventually kill them) so that you are not ONLY fighting prometheans in Halo 6. Heck maybe even the flood is used to kill the guadians… maybe cheif rides a flood bomb through space right into a guardians face like in halo 2 haha.
Now I know I probably have an active imagination and that is kind of a lot of detail but the core of that is what I kind of expected from the story. That being said I think that is the outline of a really cool story and I hope you guys agree 
What actually happend in halo 5, roughly:
- Chief did go rogue, but by rogue he basically said he wants to rescue cortana instead of letting osiris capture cortana.
- ONI, despite being a main antagonist in HTT, was not really involved in Halo 5
- Chief was perused by locke but from the opposite perspective that I had imagined.
- MDama was killed off very early, which was a shame
- The perspective of humanity on the chief was kind of lost in the story.
- Cortana did gain control over the guardians setting up a cortana + prometheans vs the world in halo 6.
… Halo 5s campaign was still awesome and halo 6 is set up to be an amazing finish to the trilogy. I just feel they created such an intriguing element to the story of ONIs behind the scenes involvement with everything and it kind of was not included within the game. I feel like including it would have given ALOT more validity to the pursuit of chief by Osiris. I also feel like the story would benefit greatly from having an ONI figure head that was villain operating behind the scenes, kind of like a emperor palpatine type roll (minus the force powers lol) I still think the story is open enough though where this can be added in. What if this ONI person was guiding these events the whole time and they are finally revealed in halo 6, MDama is dead but they could still try and manipulate the Banished brutes. Perhaps they are just looking for peace on their planet and ONI poisons their mind by saying chief is trying to stop the guardians from providing them that peace. Its gonna be a hard sell to re-ignite the locke thinking that chief has gone bad narrative but ONI could still manipulate the narrative with fear mongering to try and sway the rest of the UNSC/ humanity that Chief has indoctrinated Osiris and they are now siding with the evil covenant… wait, could the ONI boss be Donald trump ? (jk, had to)
Thanks for reading, look forward to any other thoughts. 343 feel free to borrow any of my ideas 
free of charge aha
TL:DR: I feel like hunt the truth set the game up really well with ONIs potential involvement and I wish it was included more in the final game, I feel like it would have given more validity to the motive of locke and UNSC to be hunting chief and would have made for a cool potential villain.