Find some art that is large enough to lean against the 2 walls and add something on each side.

provide a picture that includes the top of the fireplace and the ceiling so we can see what 55" looks like.

Creating a faux stone flat surface is a good idea. You could also create a flat surface with a wood cabinet with a wide "hidden" door -- one that doesn't look like a door -- and use that hidden space as storage, still adding a huge clock to the front of the door.

Alternately, you could add a couple corner shelves.

http://0.lushome.com/wp-content/uploads/2014/06/corner-wall-shelves-interior-decorating-8.jpg