I think it looks fine. Whatever you do above will draw attention away from the look you feel is too big. The logs inside will change the effect as well. I know how it feels when it doesn't look like you imagined but it will be ok.

What is the brown square on the wall? What you have so far looks fine. Skip the mantel, and add a large colorful piece of art above to balance everything out.

I personally would paint the mantle and the whole FP bump out a dark dramatic color... especially if you plan to mount the TV above the FP.