Hello, I would first like to say I have not read either of the Forerunner books, nor finished Glasslands, however I have read a lot on Halopedian, and listened to many thoughts on this forum. Feel free to correct me on anything I mistake. Now onto my speculation.
At the end of Primordium, Chakas captures a UNSC Starship and takes it to find the Librarian. Now, personally, I believe this ship to be the Infinity, as that was the first thing that popped in my mind when the captain said, “An Unknown Enemy has seized control of our ship!” Now why would he fly to Requiem to find the Librarian? She should have died on Earth when the Halo’s were fired? Well, many have speculated that Cortana has the Librarian’s Geas, or Halsey did, and passed it down to Cortana when she flash-cloned her brain. This could also explain why Greg Bear said there are two Didacts. One is the actual, alive Didact, and the other is his Geas implanted into John. This is why he has had all this “luck”, and why he has been chosen by Mendicant Bias to show his masters he has changed. This is also why him and Cortana are such a good pair, and why she chose him over every other Spartan. Their Geas direct them to. (Also why Halsey liked John so much)
Didact shouldn’t be the enemy, at least it wouldn’t really make sense for him to be the enemy, at least to John. John stopped the Array, the Didact hated the Array, but knew it was a last ditch option. Also, the Didact (Bornstellar Didact) respected humanity, not despised them. Also, when the sphere thing, (Labeled as a Cryptum) popped from behind John, he slowly lowered his weapon. Cortana also said, we know where he’s heading. This points to the Didact as well. Also, when the Elite charged John, a weapon of extreme power shot out like the beam from a sentinel and vaporized the Elite, yet not John. For one thing it never fired at John, even though John stepped out of cover out in to the open. Plus, it was a beam, not a pulse from the Light Rifle or a Scattershot. It seemed like an entirely new, long range, and more powerful weapon, yet it stood down as John passed by. Why? Maybe because it isn’t an enemy? At the end of the trailer it shows Didacts symbol turn upside down, sort of like corruption. This maybe could be signifying that there might have been a power struggle between Forerunners. (Likely as the surviving Forerunners were few in numbers, left the Galaxy presumingly. Meaning they’re without political power, low in technology, and probably have feuds with each other, what’s stopping individual factions from forming? Perhaps a Forerunner, maybe Faber, who really hates humans, decided to wait in Requiem, knowing it was near the Ark, and that Mendicant Bias would send a high value human there, and formed his own personal army, while the Didact heads the other army? It’s a possibility)
Anyway, all this is just my own opinion on what were given.