I got an idea bunny.
Fem Billy (Billie) × Steve
What if Billie and family moved in before S1. Like Billie became part of Steve's group, with Steve, Carol and Tommy. Billie and Steve gets closer, like they haven't with anyone before. Billie falls in love with him but Steve is too oblivious and little bitchy too, and thus ignores the obvious and goes for Nancy, partly becuz he genuinely likes her and partly becuz unconsciously trying to make Billie jealous. S1 party at the Steve still happens, Barb joins in too, but leaves when Nancy goes up and it's Billie who is left alone near swimming pool instead of Barb and gets taken away by demogorgen instead. But Billie doesn't die, like Will, she tries to run and hide and is succeeds and meet Will and tries to protect him.
Billie's disappearance shakes Steve too much. Makes him realize his love for Billie, and when he finds Jon took pictures of him and Nancy, instead of focusing on that he focuses on Billie's picture (with demo shadow behind) and joins in helping Nancy and Jon in finding Will and Billie. Along the way confessions between Nancy and Steve happen, how they both have feelings for each other but more stronger feelings for other people.
Billie meanwhile trying her best to protect Will and succeeding, but at the end, where Demo gets Will (like how he was found?) It's Billie instead, who protected Will and asked him to run and hide and got captured like that. She is rescued, along with Will, by Hop and Joyce, but her condition is worse.
Tommy and Carol are close with Billie, genuine friendship and they try to help Nancy, Jon and Steve too (I like Keg gang friendship too much)
Billie and Will form a unique sibling bond after. (Idk who to give mindflayer storyline becuz both of them have been flayed 😭)
I have thought of some minor plots and details too. Like the story is all laid out, I just can't write 😭