After much time pondering the very question many halo fans have questioned themselves, I feel this answer is worth a read. The problem 343i faced was deciding which direction Bungie was going in. Were the Human’s descendants of Forerunners? Or were they two separate species? While many halo fans were intrigued by the idea that the Human’s were descendants of the Forerunners, 343i needed a reason to make more games, thusly, the Human-Forerunner War was born. The concept is interesting, no doubt, but it loses that sense of mystery surrounding the Forerunners that players have had since the very beginning. After reading through various suggestions on “how Halo 4’s story should have gone” I believe what begins in the next paragraph is something you’ve never heard before, and something halo fans and 343i would have loved. So let’s begin.
Note: My idea doesn’t come out of thin air, but rather tweaks what is already in place for us. You will notice great similarities between the content we already have BUT you will notice how just a few changes makes it something you wish had come to fruition.
A long time ago, the Precursors created their first species (let’s call them The Fallen because I’d be here all day trying to think of a compelling name) in an attempt to find a species that could one day hold the Mantel. The Fallen were fast learners, innovative, and very proud of their species but relied heavily on their technology and cared little for those unlike themselves. The Precursors saw this, and believed that their self-righteousness was the final hurtle to holding the Mantel. Little did the Fallen know, the Precursors also created a second species, The Forerunner. They were creative, aware of others species not like themselves, and relied on the knowledge of the past and tradition in their growth as a race. The second born, the Forerunner, in no time became the Precursor’s favorite and were decided to one day be the holders of the Mantel. When The Fallen learned of this, their pride fueled their rage because their entire civilization and their entire existence was bent around becoming the holders of the Mantel (The Precursors had zero interaction of the Forerunner and the Forerunners didn’t even know they existed or what the Mantel was which was exactly what the Precursors wanted. On the other hand, the Precursors had interacted with The Fallen who learned about the Mantel because of this which drove their civilization. It is this reason why the Precursors believe they became so self-rightous). In this rage, The Fallen decide to wage war with the Precursors in an attempt to take the Mantel from them, which essentially means exterminating them (Their defense is, they must be eradicated to progress as a civilization across the known universe).
In the end, the Precursors have been chased to extinction but not before the last of them change themselves into a powder which would eventually spawn the first flood forms (Remember that a lot of these ideas come from content we already have and I’m just making a few changes). Much like the current story of Halo, in this new form, the Precursors are unstable and become the ravenous destructive, all consuming flood like we know them to be. Meanwhile, The Fallen have turned their search on for the Forerunners whose existence they only discovered when the final Precursor fell. When they finally find the Forerunners, the Forerunners are already fighting a battle against the flood (perhaps the flood went to the Forerunners because they knew that The Fallen would be searching for them). Not knowing what the flood was and still hell bent on exterminating the Forerunners, The Fallen engage them both and unknowingly usher in a new war (much like the Forerunner flood war from the books we already have, however instead of Forerunner they are The Fallen). The Forerunner are able to hold off the flood (like the Human’s were able to do in the books) but were heavily crippled but luckily the Flood was focusing now mainly on the force of The Fallen (sensing a greater threat and because the Flood still have the memories of the Precursors and remember what the Fallen had forced them to become in order to survive) and were driving them away from Forerunner Space (but they aren’t trying to protect the Forerunners they just are focusing on the bigger threat and will come back and finish off the Forerunners). In this time, the Forerunners decided to to build the Halo Rings (and pretty much all of the content from the Forerunner books can be inserted here but simply tweaked to reflect my story arch). For instance, they decide to build the Halo Rings, but the Didact is still fighting to build his shield worlds. He believes that they should hide from The Fallen and from the Flood while the Master Builder believes they should eradicate the two species once and for all. Ultimately, they decide to use the rings instead of the shield worlds. However, both the Flood and The Fallen learn of this (The Fallen learn of these rings a few years before their demise) and the Flood comes back to destroy the Forerunners who are betrayed by Mendicant Bias. Meanwhile, just before the flood completely consumes The Fallen, they hide away on the Forerunners shield worlds, imprisoning themselves until they are awoken again (which they know will be after the firing of the rings, because they know they have lost and there is nothing they can do to stop either the flood or the Forerunners at this point). At last, the Forerunner fire the rings and the rest is history. The Forerunners were afraid that with The Fallen defeated, they would be the most powerful species in the galaxy. Thusly, the decided to de-evolve themselves to prevent becoming like The Fallen. And of course, they started over on a little blue planet way away from anything else. A planet we call Earth. (The Forerunners weren’t aware that The Fallen had survived). The ancient Forerunners are gone completely and are replaced by the Humans. The ancient evil that waits for Master Chief on Requiem is The Fallen.