I believe the most accurate rumor out there is that the entire Halo series is a prequel to the Marathon series. Master Chief might be the Cyborg the Marathon crew finds in space.
Now I have never played any Marathon game so I am unsure how accurate this claim is.
If that's true, will we be seeing remakes of the Marathon series? In a way it would be a Halo game since you'd play as Master Chief but at the same time it'd be Marathon making it not Halo. I read in a Halo 3 guide issue of OXM that Marathon had enemies similar to Brutes and Hunters. And the Halo SPNKR Rocket Launcher is right out of Marathon.