Y'all, I've set sail on a total crack ship.
Now hear me out: a Stranger Things AU where Eleven is 15 years old in S1, 17 years old in S3. Because of her extremely (!) unusual upbringing, she acts younger than her age, so she still fits in pretty well with the Party. Mike could even have a crush on her, but in this case it isn't reciprocated.
Then the events of S3 go down, and she and Billy are thrown together in a way neither of them anticipated. After S3, they continue to be drawn to each other, though Billy resists it out of guilt. He nearly got her killed - why would she ever want to see his face again?
Obviously my facecanon for older!El is a young Natalie Portman. She looks enough like Millie that I can accept it 😉
Aight, someone write this fic, or I'll be forced to add it to my pile of wips. Ok? Ok.