You can just do a drywall build-out above the mantel, running all the way up to the ceiling, to balance the depth of the built-ins. You already have perfect lighting in place to showcase art. Another possibility is to run stone up the sides, with drywall in the middle, which gives texture without being overwhelming. You can also do a wood trim treatment above the mantel and carry it to the ceiling. See Houzz photos under "high ceiling fireplace" for examples.

What a beautiful room! Such fun! First I'd embrace the height of your room and definitely show it off with a tall fireplace feature. Whether you take the stone or marble all the way up or do a drywall build out. Here are a few pictures for you. But first decide on the style you want. Modern? Traditional? cozy? etc. I love the idea of adding a chandelier in front of it.