Carrie being publicly associated with four songs in succession, "See You Again", "Keep Us Safe", "Somethin' Bad", "All Is Well" - all having, in their own way, a definite prevailing "General Music" feel - would be tactically questionable. I think Carrie would see the need to break that pattern, and release something that would reallyl set alight the format she releases to and bases her career on. For that reason (if no other) I'd be surprised if she goes straight into "All Is Well".
If the tweet was accurate, I think it would be likely to be something new, something striking and something probably indicative of the next album, rather than a collaboration or a Christmas song.
(But, then, I'm used to being wrong, lol)