A chunky mantel running across the entire face of the fireplace would be sleeker. It's a bit hard to see what you have now in your photo. It's a beautiful fireplace! Maybe what you need to consider is painting the flanking built-ins, unless they are high quality wood, to give you more contrast.

I like the chunky mantel idea. What color would you suggest painting the cabinets? I have already painted the dark paneling that was on the back wall a light khaki color.