It's certainly evident that some previous Halo will be ported to PC at some point. But what? The entirety of the Master Chief Collection (a current-gen remaster of Halo, Halo 2, Halo 3, Halo 4, and later Halo: ODST as DLC) isn't a bad bet. Alternatively, it could be Halo: Reach -- Bungie's swan song before it moved to Destiny. Or, it could simply be Halo 5.