70 years after Halo, the humans' fledgeling planetary empire discovers a pair of alien pants that allows Spartans to teleport to other timelines...
Marathon vs Halo.
I'm not sure I ever quite thought of that before. If humans evolved the same, you'd have to explain the existence of Covenant, Jjaro, Nar, and so on in both universes, although their development took a very different path over the years 1991 (Bungie's O:DS) to 2525 (first contact at Harvest). But depending on the backstory already written for Halo 2, that may already be true.
While I'm posting, I guess I'll ask: is there any way Oni and Halo could be the same universe?