Well if we're throwing out plot theories I guess I'll throw my hat in the ring.
I think it's essential to recognize what little direction we know 343i is taking with Halo Infinite, that primarily being creating the game as a "spiritual reboot" of the franchise, as well as it being an accessible to all players new and old. To me, this is 343i beating around the bush that this is a full on soft-reboot, comparable to what The Force Awakens was to the Star Wars franchise. So unless, the art, music, and marketing direction to the game is all gloss for something entirely different than what they seem to be signaling, I'd assume they are throwing as much out the window from Halo 4 and Halo 5 as possible in an attempt to make a entirely new chapter to the Halo franchise, and what I think will ultimately be the true first game in their Reclaimer Saga. With that being said, and considering the amount of speculation that would properly conclude a second trilogy - despite this not being the case, we should outline what about 343i's installments are likely able to make it into Infinite, as well as what is likely to be cut.
From what we already know, the key elements of what makes a mainline Halo game a mainline Halo game seem to already have been revealed, those being the Master Chief, Cortana, an actual Halo Ring, and the UNSC involved in some galactic conflict, presumably with The Created or some other evolution of an AI conglomerate. Notably, the Covenant have not been outright confirmed yet, however I think it is safe to say they will be involved at some capacity considering their importance to the gameplay sandbox of Halo. The same could be said about the Prometheans, but seeing how they have been met with mixed to negative reception from the fanbase, they are subject to being significantly changed, reduced, or completely written out and possibly replaced by a general Forerunner Sentinel enemy faction.
Beyond this is pure speculation, but I believe 343i will significantly be cutting down on the unessential characters, stories, and plot elements introduced in Halo 5 to make way for a totally new adventure. This would naturally mean benching Fireteam Osiris, Blue Team, the Warden Eternal, the Arbiter and the Swords of Sanghelios. The Guardians, seeing how they are instantly overpowered and virtually unstoppable, will most likely be retired to the background and only mentioned outside of the limited setting of a story based around a Halo Ring. Maybe a Guardian was called into the battle the UNSC is said to have lost in the beginning of the Discover Hope trailer, incapacitating the UNSC's fleet and destroying a fragment of the Halo Ring as collateral damage from either the Guardian's destruction or an attempt at destroying the Guardian itself, however I think this will only be hinted at through exposition or visual storytelling if that is the case. Otherwise, the set up is going to be incredibly reminiscent of Halo: Combat Evolved (as confirmed by Frankie) and we will be entering the aftermath of this fresh conflict with only a limited selection of iconic or fundamental characters, such as those aboard the UNSC Infinity like Lasky and Roland (...and Palmer, I guess).
For more general speculation based on the trailers and other information provided by the game's marketing, I think that game will start out small and build up to become something much larger. It's confirmed that the cutscene from Discovered Hope will be one of the first things you see in the game, so it's safe to say that's the starting point outside of the possibility of an opening text crawl like in ODST. From here, story will most likely be pretty light and mostly follow Chief and the Pilot's journey to get home, which will eventually evolve into a much bigger adventure (as they always do). I don't think it's a coincidence that the first character you encounter is a pilot with access to a somewhat functioning Pelican, and like Foehammer from Combat Evolved he will be your ticket to traveling around the Halo Ring. However, I think his involvement will be much more directly tied to the traversal of level design, him being incredibly comparable to Morpho/Pequot from Metal Gear Solid V or Cere and Greez from Jedi: Fallen Order, acting as your method of transportation between different regions/levels of the Halo Ring. 343i has indicated that they want Infinite to be more expansive, which opens up the possibility of a more open world design.
Back to the game's progression, it seems after encountering the enemy - who are most likely the Covenant under the command of The Created who have either formed an alliance or view the AI as some extension of their Forerunner inspired religion, serving them as they once did the Prophets - the Chief and the pilot will return to the surface of the Halo Ring, where UNSC survivors of the battle have taken refuge and are continuing the fight on the ground outside of a potential Guardian's reach. If a Guardian did in fact cause their loss, many ships are likely to have crashed on the surface of the ring, but nonetheless there will be many survivors and familiar characters to reconnect with. Now the final piece of the information we were given, and potentially the most significant, is the Chief's encounter with the "Blue Light." Cortana notably narrates this section of the Discover Hope trailer, but it is also possible -
backed up by audio files extracted from the same trailer - that a piece of Cortana has been preserved in this Halo Ring, and reconnects with the Chief early on in the game (there's no way this is a late game spoiler). Information she could have obtained from somehow living within the construct could further advance the plot, as well as add to your "crew" and expand location for you to explore on the Ring.
The rest of the story is entirely up in the air, but I think this will ultimately be the foundation of the game's plot: After a devastating loss at the Battle of Zeta Halo, the Chief and the Pilot attempt to reunite the scattered UNSC forces of the Halo Ring and either get back in the fight or get home. Along the way, while exploring the new Halo Ring, he encounters a fragment of Cortana, reconnecting and recruiting her to help aid their fight against the Covenant and the Created. Herein lies what I think may be another larger plot point, which could be collecting more fragments of Cortana, effectively making the game about rebuilding the beloved character. The Created may evolve past their initial purpose in Halo 5, and the Flood could even return and corrupt them, turning their quest to bring order to the galaxy through the Mantle of Responsibility (Forerunner imperialist doctrine) and the Guardians into one that will protect the spread of the parasite with no contest. Like I said, it's anyones best guess, but I do think they will be getting away from the previous two games story wise and start off a new chapter will a very simple but effective first step.
Edit: I forgot to mention that The Banished faction from Halo Wars 2 may eventually play a role and help reintroduce the Brutes as an enemy, however I think they exist as their own separate entity and would further complicate a rather clean and simple reboot since they may not side with Cortana or follow the old religion of the Covenant.
