I believe this would make the visual novel more expressive, conveying emotions more effectively than words alone and adding depth to the characters personalities and add more immersion and variations that would make the experience more variable. Not every character needs these variations, but the main ones should have them, expressions like happiness, anger, sadness etc.
Many visual novel have such features, so i think this would be good and suit the world also. Considering the artstyle it will be hard and might divert recourse somewhere but it's just some thought.