According to the findings of SteamDB, a reputable site that keeps track of changes in the Steam database, listings for Halo 5: Guardians and Forza Motorsport 5 have been discovered within the Steam database.
This seems to imply that the games might launch on Steam at some point in the future (possibly alongside, or after, a Windows Store release)- at the same time, however, it is important to remember that not all Steam database listings pan out.
There are, of course, still some problems- Microsoft’s Xbox games are only available on PC via the Windows Store, which is loathed by just about everyone who uses it, and they are tied to the UWP format.
Microsoft’s PC gaming initiative has begun to bear fruit a little, with the last few PC ports of their games actually being fairly good.