[This is the next summer, after the shows events]
Basically, Dipper and Mable were exploring in the woods after they heard a loud crash.
They eventually find the crash sight.. Low and behold, its Bill.
Though, he doesn't seem to remember them.. And he doesn't have his hat or tie. His iris is even a circle now instead of the cat-like one he had before.
Mable thinks hes changed, Dipper isn't a fan that hes back though.
Bill's confused, yet he seems kind.
Mable thinks, what if they sang together?
Dipper agrees after some bargaining, and starts the music on his phone.
Now, with Mable singing with Bill, what'll happen?

A messy sketch of what it might look like-
I forgot to draw Dippers phone, but you get the idea.