Hi there! very cool piece. I like the vibe. If you want to get a more realistic result maybe you can add more ambient shadows and look for references in overcast lighting to see where to put light and shadows. You can totally add more depth so things will feel closer and farther, also by using deeper shadows or atmospheric light farther away. If you're looking for a flatter style which doesn't have realistic depth and shadows you can still use these principles in a stylistic way. You added some shadows on the shirt but not the arms or other things so maybe you can start from there. If you have any particular reference or style you want to achieve maybe you can share it. :D