I tried my best working with the lines you had. I'm sorry to say though, without heavy modification, the lines are extremely boring. I don't mean to be harsh, but it's the truth. A generic smiley (or frowny in this case) is usually interesting if it's very small, like emotes, otherwise, they're just about the plainest thing you could draw.
My tips would be to first of all, get rid of the circle; either use a custom shape, or add features, like cheek bones, etc, to the the existing circle, and second of all, add more detail to the inside of the face; as it stands, he looks grumpy/sad...expand on that with fitting eyebrows, etc. And lastly, maybe something other than yellow