An interesting idea I had for a sort of inversion of the story.
We all know the premise. Its the title. The guy she was interested in wasn't a guy at all because Aya thought that Mitsuki was a man at the CD shop. What if it got changed around a bit and CD shop Mitsuki is the girl and school Mitsuki is the "guy"?
In this AU Mitsuki is a trans girl who has to boymode at school. Aya sits next to her but doesn't really know her, until one day some kind of circumstance forces them to interact and Aya starts to develop feelings for her, which is weird for her because she's not usually interested in guys, same way it happened in the original.
Just a random thought I had, its not very developed.