Add musical instruments with playable notes, similar to some piano-playing games on Roblox.
The most feasible instruments would be some kind of string instrument, or perhaps ocarinas and flutes.
In my opinion, this would greatly enhance the immersion in the game. Imagine meeting someone and having them play (with their own hands, real music, not a pre-recorded track) the opening theme from Kimetsu no Yaiba for you!
Also, based on my limited knowledge, I think implementing it wouldn't be that difficult. If I'm wrong and have underestimated the programming involved, I apologize.