Be careful with drywall around a fireplace. General codes preclude it within 12" of the fireplace opening, you'd need to use non-combustible sheathing or simply stucco coat on the brick.

I'm with Brenda: the back brick wall would actually be pretty cool if it weren't for the brick overload at the piers and fireplace. The piers and fireplace need to get calmed down to make the rest of the brick pop. Some horizontal lines would also help de-emphasize the verticality of those piers and inner space.

My instinct is to coat the two brick piers with one finish (concrete or stucco) jointed horizontally @ ~24" o.c. and fill the space between them with 1/8" steel plates on four through bolts and a spacer reveal, also jointed to aligned with pier joints. Add a stone hearth and mantle to finish it all off. From there, it's a question of colors, and surface textures. Should not be that expensive to do. The stone would be the big cost and you could forgo that with a parge on the hearth and maybe a rough hewn (reclaimed wood?) or even glass slab mantle. And in an ideal world, you could sand down the remaining exposed brick to give it a more uniform color and rougher texture to contrast with the smooth fireplace finishes.