I love what you've dreamed up here and I especially like that I can see where the game can go. As a player, I wish I had more insight about why I would add certain things to my potion and some way to visualize the risk/benefit might pay off.
Like another commenter said, I loved seeing the ghosts come back and tell me if they passed or failed! The NPCs and what they had to say were fun and compelling.
I love the idea of the potion store and I think it was very clear the basics of what you were supposed to do.
Maybe the UI could have a "progress bar" that represents a range of probability a NPC a potion will be "successful." Maybe there is a certain ingredient that makes that range tighter. This way the player has to balance what to add. "Should I make the range more accurate? If I do there is a chance it will backfire."
Just my outside, random thoughts.
KEEP MAKING MORE GAMES!!!