The Elites are a race of saurian bipedal aliens that originally were the foes of humanity as the warriors of the Covenant, but later became our allies after the Great Schism where they were betrayed by their leader. Elites’ culture is easiest compared to Feudal Japan, with a number of large houses, each headed by a Kaidon, controlling land and lesser families living within their area of jurisdiction, much like a Feudal Lord. Their culture is based around a system of honor to their house and ancestry.
They train from an early age as warriors and scholars to uphold the honor of their household and their Kaidon. The greatest honor a warrior can receive is a noble death in battle, which leads them to see the use of medics or guerrilla tactics as dishonorable. When the Covenant was originally formed this strict militaristic way of life easily translated over to the life of a soldier, while Kaidons became Shipmasters in charge of everyone on their cruiser. The Elites regard the ancient race called Forerunners as gods, and revere their technology as sacred.
Throughout their battles with humanity they began to see the honor that humans practiced and some began to even regard them as equals. Thus when they Great Schism transpired and they learned how the Forerunners were not gods, they joined with humanity to fight off what remained of the Covenant.
The Fallen are a bipedal race that we believe to be insectoid in nature. Now as for their original culture little is known, though their current way of life still clings to its previous grandeur. We know that the Fallen were divided up into different houses which were independent, though kept united and peaceful by the House of Judgement. This house was supposed to divide its members amongst the other Houses and, we assume, make sure each house kept to a certain code of honor.
The race at some point was uplifted by the Traveler, but the Traveler then left them when the Darkness once again caught up to it. The Fallen’s great civilisation was then destroyed by what they now refer to as The Great Whirlwind. The remnants of the race began a bloody conflict over the ruins of their past glory and began worshipping their own technology, and created the Servitors, in the likeness of the Traveler, to provide them with Ether, believed to be a synthetic replacement for light.
Eventually the houses came to a tentative peace and went out across the solar system in search of the Great Machine, as they called the Traveler, which is when they came across humanity. They now operate by a perverted sense of their original honor, with those deemed dishonorable being docked of their arms, receiving limited rations of life giving Ether, and being used as cannon fodder, while those in higher casts command ships and grow huge on Ether supplies.
The clearest similarity between these two races is this strict code of honor. Both look up to and receive commands from leaders of houses who command ships, and are punished for fleeing in battle or otherwise doing something seen as dishonorable. Both races also worship the technology of preceding advanced races, the Forerunners for the Elites and the Traveler for the Fallen. In both universes this higher race ends up choosing humanity to represent it. The Forerunners passed on the care of the galaxy to humanity, while the Traveler stayed and protected humanity (whether willingly or not is a different issue).
In Halo 2 and 3 Thel Vadam, in the honored position of Arbiter, united the Elites and joined humanity in the fight against the Covenant. During House of Wolves we learned of a similar figure foretold to do the same thing for the Fallen called the Kell of Kells. We were also introduced to Variks who is allied with humanity and wishes to reunite his race and reclaim their former glory.
With these similarities, and the common creator in Bungie, I believe that in the Destiny sequel we may in fact unite with the Fallen against the darkness, just like the Elites did in Halo. Variks has shown fondness to our Guardians in the past and in the Taken King, during the Wolves of Mars quest line, it is hinted that Variks has been working towards his own little project of uniting these Fallen houses and discovering, or perhaps becoming, the Kell of Kells.
Of course none of this is certain, and is but my humble opinion. However,as a lover of their interesting race I hope to one day play a Fallen guardian. If any of you readers out there have anything you’d like to add, multiply, divide, or subtract from my theory or if you have a theory of your own, go ahead and hit up the comment section and let me know!