Apparently, there were previously long-standing plans to launch an extensive collaboration between Call of Duty: Modern Warfare III and Halo, where several of the maps from the Master Chief's sprawling universe were supposed to be introduced in multiplayer.
This is according to information shared by Call of Duty leaker TheGhostofHope, which includes claims of screenshots that have not yet been disproved. Exactly why the whole thing fell through is still unclear, but given that Microsoft now owns both brands, a new, future collaboration between the two may not be entirely unlikely. Who knows, though.
Could you imagine a crossover between Call of Duty and Halo?