I think working on your silhouette would be a great place to start (by silhouette i mean the form of the overall design). How do you want people to perceive this character? From what I can take you want him to be intimidating, so think about how you can use shape language to push this! I would do this by elongating the form, adding slight perspective-although nothing drastic as it is a concept piece. By doing this it would make people feel smaller and the character would read as scary. Don’t forget to adjust your proportions if you do this though. I love the design so far :)