Okok I thought of something. And yes, of course okok is a word!
What if... Halo and Marathon take place in the same universe? Now something tells me this has been suggested before. And I have a feeling that good points were brought up that make it seem unlikely, such as the human race's inability to perform basic stunts like teleportation in Halo, the fact that the Grunts don't speak in metaphor, that no Covenant looks anything like the pfhor fighters...
So what if it's the same universe, but alternate time lines? That's also been suggested. But it doesn't really unify the games any more than this turns all games ever made into a single one: "all games are in the same universe in different timelines!"
But...
What if the fork in the timeline that results in either Halo or Marathon, occurs... in Pathways into Darkness? It takes place before both Halo and Marathon, and it is accepted as Maraverse Canon. In PiDVerse, the Jjaro visit Earth - to take care of the W'rkncacnter. This could take place after the Fork, and in the Maraverse. A visit from peaceful aliens would be something humanity benefited from, right? One would assume that they taught us things, and that this eventually led to us figuring out teleportation technology sooner than in the Haloverse, in which the Jjaro never come to Earth.
In that case, something that happened after the Fork and in the Haloverse would have led to the extinction of the Pfhor, and the Nar would evolve into Grunts and join the Covenant. And one thousand thousand slimy other changes.
Someone come up with a clever theory about what exactly the Fork is. It can't be that the player fails - you'd think someone in Halo would have mentioned that enormous God that awoke and wreaked havoc all over the entire universe. ;)
