I would place a long vertical piece of art or possibly a metal sculptural piece.
Although the wall is quite tall you could also use a piece of art that is still vertical, but having the top level with the top of your door casings. Check out art.com for prints and posters. I know you didn't ask but I would move the two candlesticks to the left and place the two other pieces on the right with the taller of the two on the right, ignore the small areas that you now have them. Enjoy, and have lot of fun with that wall and mantle!

Hi, Dash, this caught my eye, as my FP is similar, although my ceiling isn't as tall.

I painted the bump-out a chocolate color, and hung my favorite picture. I didn't want a mirror, since it would only reflect the ceiling/ceiling fan. I'm not a fan of giant clocks, but that's just personal preference.

Here is my FP: