Your wood mantle is far too small for the size of that wall. If I were in your shoes (and I have been), I recommend you consider having a much larger surround built.

I could see molding to help balance height of ceiling and the large windows. It would be even better with chests/shelves or bookcases on either side.

you haven't shared your style or interests.

For something Now, fun, inexpensive and easily removed when you find that 'just right solution',

I suggest a big kite.