Unlockable Radio Cassettes
I noticed that there's a lot of quest music tracks that aren't on the radio. Examples include the boss themes, which, in the case of Crimson Cauldron and Jumbotron, are absolute bangers.
I feel like there should be an extra reward at the end of a quest, once you've gotten all the cosmetics. Upon collecting all cosmetics and earning an S tier after the fact, the reward chest at the end drops a cassette tape with the previously inaccessible tracks.
It would be a nice send off now that you're finished with the quest for good (no more rewards or anything else to go for), and would give the player one last challenge.
Not to mention, being able to listen to have these hidden tracks play in your dorm would be a nice touch, a sort of trophy for completion.