get a thick plant of wood with a live edge that hanged over at least 1.5 inches

I would think about continuing the stone work to finish the mantle.

i agree it needs another layer of stone, that is capping across the top. the other mantels wont work they are not accounting for all the depth you have. i would actually add dry wall and stone up and add a mantel but here are two option

agree less is more !

My fireplace is very similar. Mason finished it with 3 large capstones as suggested by Flair Lighting. Of course, I broke the Houzz rule and mounted the tv above, but it works for me.

Go for the live edge wood for a rustic approach or the thick wood mantel for a sleeker contemporary feel.

Not seeing the rest of the room. Consider this mission furniture influence.

http://www.cowichanwoodwork.com/fireplaces-built-ins-and-millwork/fumed-oak-fireplace-mantle