I'm surprised no one mentioned the Vex when talking about alternate timelines.
The Vex are the enemies that look like robots (they are actually a liquid race of white "mind fluid" that use the robot suits as mechs). Their main motivation in the game is this: they found a way to see the future and can see the eventual extinction of their species, so they are exploring alternate timelines in order to escape their inevitable death. They convert entire planets and moons into giant Vex computers to help them 1) explore alternate future timelines to find one where they don't die (they do this partially by creating hyper-accurate simulations of reality, and partially by actually traveling through time), 2) try to rewrite the rules of reality so that "the Vex" are a fundamental part of the laws of the universe and therefore cannot die.
The very first raid in Destiny 1 explored this narrative by sending the players into the Vex's past and future. In the past, the world is a dried out desert, but the Vex have shiny, expensive suits of armor. In the future, the world is overgrown with plant life, but the Vex's suits are worn-out and made from cheaper metals.
The tragedy for the Vex is that they still haven't discovered a timeline where they survive. Part of this is because the "Light", the power of the Traveler and the Guardians, is a "paracausal" power (meaning it is beyond causality, outside of normal time) and this paracausal "Light" prevents the Vex from accurately simulating or accurately seeing the future.
There are obviously a lot of echoes to Marathon there. The Vex are a kind of mix between the player in Infinity and Durandal throughout the whole series. Interestingly enough, this means that the players and Traveler in Destiny kind of take the role of the Wrkncacnter, by obscuring the future and effectively being a force of chaos to the Vex.
