Before there was Shattered Skies: The Morning Lights (albeit not much before), there was Stars Above, the Lucky Star/PMMM crossover we didn't know we needed.
https://archiveofourown.org/works/979513
It spawned two side stories, telling what happened to some of the other Puellae Magi in the last days of their world, and a database of the Demons that are the main story's big bads (similar to the Shattered Skies side-story Viluy's Database).
AshenDream and BHS Wrote:Two very different worlds collide: the world of ordinary high-school girl Kagami Hiiragi and the world of Puella Magi Homura Akemi. An impossible bond begins to form between them, but will the demons of Homura's past ever let them be friends at all, let alone anything more? Kagami/Homura, Tsukasa/Miyuki. Rated M: violence, yuri shipping, sexual content and themes, disturbing imagery.
https://archiveofourown.org/works/979513
It spawned two side stories, telling what happened to some of the other Puellae Magi in the last days of their world, and a database of the Demons that are the main story's big bads (similar to the Shattered Skies side-story Viluy's Database).