The Halo: Combat Evolved portion of the five-game Halo: The Master Chief Collection—which comes in the form of 2011's 'Anniversary' remake—recently concluded a couple weeks of beta testing, and going by the February wrap-up at the Halo Waypoint website, it was a very productive event. A number of outstanding issues were resolved, an updated crouch/movement system was added because the legacy system "wasn't a good experience on PC," and players seemed very happy with the experience.
That's not to say Halo: Combat Evolved Anniversary is ready to load and fire on Steam, necessarily, but a new trailer released today could be taken as a hint that, maybe, it might possibly be coming soon. The video portion is new (and will apparently provide the backdrop for a new Master Chief Collection main menu) but Halo fans may recognize the audio as coming from the opening cutscene in Halo: Combat Evolved.
The video could be just a nostalgic nod to the first-ever Halo game (but the second in the Master Chief Collection, which is following the story chronologically and thus started with the prequel, Halo: Reach). But it's such a clear reference to Combat Evolved's opening sequence, and combined with the imagery of the Master Chief emerging from stasis to take up the fight, it's not hard to read something more than that into it.
A relatively soon-ish release for Combat Evolved wouldn't be out of whack with the planned Master Chief Collection timeline: Halo Reach arrived on December 3, 2019, and CE, Halo 2, 3, and 4 are all expected to be out this year. At one per quarter, early to mid-March would be a reasonable time for it to show up.