Not /co/ (well, I assume there’s at least one /co/ adaptation somewhere), but in Maurice Maeterlinck’s The Blue Bird, there’s a whole chapter where the protagonists go to The Kingdom of the Future, and it looks just like the portion of Soul’s trailer where it’s basically heaven with fluffy comfy clouds and shit, only full of a bunch of souls of children that haven’t been born yet and maybe a couple of angelic people overseeing them. I think the main difference is that the Kingdom of Future is overseen by Father Time, and the kid souls all sorta already know their futures but are aware that they’ll forget the knowledge of who they’ll be once they get to earth and are born as babies. One of the kid souls for example tells the protagonists that he’s gonna grow up to be an inventor. Otherwise it looks the same as the great before in Soul.
Also Osamu Tezuka’s Princess Knight series does the exact same thing, only the main focus on that version of the “great before” is put onto assigning the unborn baby souls their genders- boys get given a “blue heart” by some angels while girls get a “pink heart.” The Princess Knight story is kicked off when the main character is given both colored hearts before being sent to earth by some dick cherub that either fucked up or just wanted to see what’d happen, and that kinda sets up Princess Knight’s gender confusion for the whole rest of the story.