The launch of Halo: The Master Chief Collection on PC was received very well, even though Halo: Reach is the only game currently available. 343 is rolling out the Halo games included in Halo: The Master Chief Collection over time, starting with Reach and moving forward chronologically. It seems 343 is ready to move onto the next chapter of Halo with Halo: Combat Evolved PC tests set to begin in February.
In 343's latest PC Master Chief Collection blog, the developers over an update on Halo: Combat Evolved. 343 says that Halo: Combat Evolved PC development has progressed to a state where the team is ready to test several key parts of the game. These include validating dedicated servers, peer-to-peer-connections, PC storefront crossplay, UI customization, and seasonal progression systems. These will be tested via single-player, co-op, multiplayer and so on via the upcoming flights.
Testing flights will start for 343's partners as soon as late January. Invitations for Xbox Insiders will have to wait until February, just to ensure the test flight's stability. To get invited, PC and Xbox users need to download and run the Xbox Insider Hub app. From the app, Insiders can register for upcoming Insider content but there's no guarantee that everyone will be invited.
In addition to Halo: Combat Evolved, 343 will also be testing some changes to Halo: Reach on PC including the ability to crouch while moving. More improvement and updates are said to be on the way, but no other specific changes to Halo: Reach are mentioned in the update.
Halo: Reach flight testing took place well over 6 months prior to the game's eventual launch on PC. It's unclear if 343 plans to take a similar amount of time before it officially releases Halo: Combat Evolved, though it can reasonably be assumed that it will be some time before launch. Given that kind of pacing, it's now very unlikely that all of Halo: The Master Chief Collection will be released on PC prior to Halo Infinite's release later this year. 343 may even stop at Halo: Combat Evolved until 2021 just to make room for the next major Halo game, which is already confirmed for both Xbox and PC.
Halo: The Master Chief Collection is available now on PC and Xbox One, though only Halo: Reach is currently playable on PC.