Bungie said something that Marathon and Halo take place in the same universe, but in different times and places (different galaxies?).
I've heard a lot of speculation that the planet the Chief is drifting towards is the Forerunner homeworld. It has to be home to some advanced/post-industrialisation civilisation, because there are city lights on the planet.
I don't think it could be Onyx, because wasn't it completely destroyed? Sentinels too? I haven't read the book since it first came out, so I can't remember exactly.