It would have to be believable, but it would also have to maintain the characters, locales and general story that Bungie and Nylund thought up.
Since bungie decided to alter existing canon, they could’ve just…made noble team spartan-IIs, and have them more involved with the rest of the spartans/plot. there are easily 6 unused spartan-IIs that they could’ve used.
They take 6 of the spartans that were originally in red team and make them noble team. It could open up similarly, noble team is on reach, the com goes offline, they investigate blah blah. They neutralize the covenant scouting party thanks to Jorges sacrifice. However, the “real fleet” does not arrive instantly, there is a slight lull. Since the covenant know about reach, humanity knows that their time is running out, so they organize operation red flag. There is a scene where all the admiralty (including lord hood) plus keyes and halsey meet to devise the plan in some grand meeting room on reach. Noble team joins the rest the spartan-IIs aboard the pillar of autumn to start the mission, because they know that if they dont do something crazy and desperate now, reach and eventually earth is doomed. So they formulate operation red flag and their attempt to capture a prophet to use as a hostage, because they don’t know when the whole covenant fleet will arrive, so they gotta hurry. They assume it will be a week or two because that’s how long slipspace travel takes but…
While aboard the POA, preparing to slip into covenant territory, the entire fleet of particular justice warps to reach. Turns out the covenant fleet found what they wanted with the scouting party noble team was fighting earlier(forerunner artifacts) and they had already set the fleet to reach to recover them and burn the humans.
So the real fleet arrives on august 15th, just like in the game. Now, noble team drops to the planet with the rest of red team just like in the books. Once they land noble team splits up from red team and does pretty much the exact same stuff they do in game. The “real” fight lasts 2 weeks just like in the game. Obviously red team and blue team would be fighting the full 2 weeks vs the 1 day they fought in the book, so they wouldn’t all die the first day.
That makes noble team feel a hell of a lot more important already since they have a connection to master chief, captain keyes, cortana and 16 other spartans.
In dr halseys journey it is noted that on aug 27th, right in the middle of the battle for reach, the spartans meet dr halsey to discuss the state of things. They talk about operation red flag and such. They discuss how desparate the situation is. Note that this is before halsey discovers that the artifact the covenant are looking for contains the coordinates to halo. This could be used to set up the hopelessness and desperation. They have no plan and reach is going to be turned to glass in a matter of days. everyone more or less thinks they are all doomed.
So after parting ways you partake in levels “the package” and “pillar of autumn”. It is revealed that the main inspiration for the covenant attacking reach was to get the data that dr halsey and cortana manage to get from the artifact. As spartan-IIs, noble team already know who cortana and dr. halsey are and you know just how important her discovery is: it is a glimmer of hope for the future, even if reach does fall. For the last mission you are to meet with the other spartans at the POA with the package, then you are to go with them to wherever the coordinates lead (halo). Of course the entirety of noble team dies doing their mission and almost all other spartans are (presumed) dead at this point too.
When you reach the POA, the only spartans there to greet you are master chief and linda. Chief tells you that freds team is on their way. But the covenant decide to send waves of enemies after the POA, and in the ensuing battle linda gets severely injured (basically, the injury she received at the end of TFOR is going to happen here instead). Emile dies in the mini-mac.
Keyes says the’re out of time. They cant wait any longer and need to get out of there, they can’t wait for red team. Chief carries linda body aboard the pelican and being the selfless hero says he will man the cannon (he wants to wait for fred team anyways, even if he knows deep down none of them will make it off the planet). But Noble 6 basically forces chief to let him stay behind, saying “She(cortana) chose you for a reason…I have the gun.”
Extremely similar to the current game, but it feels a heck of a lot more connected to the grand story. You feel more important because you have an actual connection to established characters.
Or even…noble 6 could have been a spartan-III still, so through the game he has to prove himself to the “superior” spartan-IIs in noble team. And while most of noble team keep to themselves, emile would be the one to voice how he doesn’t like having a “weakling” on their team. And he would be pretty unhappy about cortana choosing noble 6 to carry her over the rest of the team, as he doesn’t consider 6 worthy to carry master chiefs “girl” (master chief of course being someone all spartans look up to). But in the end they all are happy to sacrifice themselves for noble 6. Making their deaths, and your character in general more meaningful. Bungie said they referenced the seven samurai as material right? Remember how the younger samurai was trying to be as worthy as the other much older samurai? I see it similarly here.