Halo is one of the most revered first-person shooters that doesn't have much of a presence on PC. The original, Combat Evolved, was ported to PC in 2003; Halo 2 helped launch Games for Windows Live in 2007. Besides those two, Microsoft has kept mainline Halo confined strictly to its consoles.
In recent years, Microsoft has made a significant effort to blur the lines between Xbox and PC gaming. With the promise that all future first-party titles are Play Anywhere (meaning that they're bought once and players can freely switch between PC and Xbox One versions), we have implicit confirmation that Halo 6 will launch on Windows 10 PC.
However, we might get other Halo games on PC even sooner. Recently, Microsoft shut down community modding project "ElDewrito" in order to protect its intellectual property. It's standard corporate stuff that we've seen a million times. But, the end of Microsoft's statement is surprisingly frank: "As we look ahead, we're very excited about the prospects of an official classic Halo experience making its way to PC." Xbox Head Phil Spencer took to Twitter to specifically call out that quote as something to pay attention to.
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.
Whatever it is, we know that Microsoft's intentions for Haloand PC aren't exclusively forward-looking; there seem to be plans for past Halos too.