I do have wonder if they're going to reprint some statblocks from other books. I could see them not reprinting the guardinals from this book, as the three here aren't particularly iconic like the leonal and the ursinal, but the lantern, hound, and warden archons are pretty iconic. Of course, as you said, finally defining the CG Celestal race for 5e would be very nice as well!
(As for other books, anything outside of MoM is fair game - I would very much like to see the gem dragons finally promoted to the main MM, for example).