I think you need help from a professional

I think you mean the surround because it doesn't look like you have a mantel. It would be helpful to see the whole wall.

That said, the sky's the limit for what you can do with this fireplace. I suggest you start a fireplace ideabook to get a sense of what you love. You can start here: https://www.houzz.com/photos/fireplace-design-ideas-phbr0lbl-bl~l_36794

The proportion of the marble surround is not quite right, which may be why you don't like it overall. Consider a thinner stone surround with another painted wood mantle to frame it in. Add depth to the wood mantle to give the wall some visual interest and dimension.