One idea could be a story where Mash teams up with an unlikely ally from a rival school. They start off not getting along but through various challenges, they learn to work together and combine their unique skills.
Another option could be a student from a different magic school who is jealous of Mash's popularity and unique abilities. This student could use dark magic to try and bring Mash down.
Well, 'old black magic ost visual novel' might be a niche visual novel. The 'old black magic' part could imply that the story is set in a world where ancient and perhaps forbidden magic exists. The OST would be an important aspect, as it can create the right mood for the various scenes in the visual novel, whether it's a spooky incantation scene or a tense encounter related to the magic. Without more information, it's hard to say more precisely.