I feel like the song Darkest Desire by Dawko really fits the Forgettable AU! It might be because of the speaker's methodical speech patterns or how we see that same speaker falling apart at the seams the longer the song goes on. Either way though, I can't help but think of the AU whenever I hear it.
Lyrics in the song I find to fit VERY well + explanations as to why I think so: "What's in this cassette?" 'IT SAID THE THING!! IT SAID THE THING,' I scream as they drag me away.
"Everything's coming together / Now erupting in terror / It's glowing forever / The fear is growing / Never confided that one day / I'd like to be invited outside of this domain" This entire part just seems like one of the cryptic messages Gaster might send (in this case through the cassette). I feel like the "Everything's coming together" part especially sells that all of this is just predestined to have Windings make the same decisions to create the outcome of Gaster and allow the events of Undertale and Deltarune to take place!
"This reality, my mentality / Everything changes so rapidly" Wingdings becoming aware of things he was never meant to and subsequently sealing his fate...
"Dancing with my deepest dark desires" Maybe Gaster, maybe the Player (are we going to be involved in the LORE???), a hint of Deltarune sprinkled in with the "deepest dark" mention. All around good line.
"As I fulfill my darkest desire" Similar to the previous one, but with some heavy implications. This is the first bad decision in a long line of them that will eventually lead to his demise.
"Every turn returns concern / But my curiosity is a growin', it keeps goin'" This line reminds me of the short comic where Papyrus and Gaster are standing over Wingdings' grave and Papyrus asks why Gaster/Wingdings did everything he did and Gaster responds by saying he wanted to see what would happen. If I had to describe the entire Forgettable AU plot in one sentence/phrase it would be curiosity killed the cat, because that is what fueled a lot of Wingdings' actions and it eventually killed him.
"Your soul and my soul collide" Probably right after Wingdings falls into the core. Gaster and Wingdings finally become one because they have reached the point in time where they are now the same person.
Thanks for listening to my thoughts and ramblings on this, and have a nice day! (This is a sign to take a sip of water and eat if you are hungry.)