Hi! I would first start by looking for something tall to go on one end of the mantel, and go from there. Height differences will make this area more enticing! here are a few ideas for you:

http://deciture.com/shop/decor/pier-1-imports/black-metal-lantern-24/

http://deciture.com/shop/decor/pier-1-imports/black-metal-lantern-14/
or

http://deciture.com/shop/decor/pier-1-imports/metal-nailhead-lanterns/
(would be nice with colorful candles)

http://deciture.com/shop/decor/world-market/red-velvet-plum-blossoms-set-2/

http://deciture.com/shop/decor/world-market/honey-bulb-rattan-coil-vases-tall/

Hope this helps you!

Well, this is what I did to my fireplaces! The first one mimics a painting of mine ("Mother May I"), the next photo is the "before" and "after" of the fireplace in my master bedroom I tiled.

Can you add another ceiling can to complete the symmetry? Can you swap the mirror for a colorful painting? (I like the look of art that leans against the wall--then it can be swapped occasionally as your mood/design ideas change). The photos really clutter the mantle--tone that down to a few important pieces that provide punch and make it look less crowded. I see a colorful painting and something tall and off-center in front of it, like a topiary, or a ceramic/glass vessel, all of which is placed between the two wall sconces. The white against the wall and contrasting the marble surround?--All good!

Here are some additional pictures of the room, so you get an idea. I had posted a few in a thread below as well.

I think your fireplace is lovely, but perhaps too symmetrical. Maybe trying something like this http://www.thepickygirl.com/wp-content/uploads/2012/03/fireplace2.jpg to break up the symmetry. I also agree with another can light above the fireplace for a little more balance.