Keep the items sparse & simple on the mantle. Think about painting the walls on the upper area the same color as the ledge to make that less prominent.

What color built ins? We have lighter oak floors, white trim, maple kitchen cabinets.

Getting out of my comfort zone to recommend you a color. Can you see this room from the kitchen? Then maybe match the cabinets.

I would have a couple of good local cabinet makers in. Once you select one, he/she can even make up some sample pieces with different finishes to decide from.

I am not sure what those dark knob-looking things are on the walls above TV and dark chest. I love your fireplace! Take everything off the mantel, it is too busy. Hang the clock lower or rest it on the mantel...it should be below the crossbar. If it is in your budget, add built ins and mantel shelf to match the kitchen cabinets. If not, add a giant painting over dark chest. In top tier niches, highlight just one large item that will make a statement.

If you want your fireplace to be the focal point of your room, and rightfully it should be, then move your couch and chairs facing it. With that said, move the fish tank to another location in the room. The TV will compete but if you want it with the fireplace, mount it all the wall high, and have an interesting small table below it. On the other side, put a narrow bookcase or plant. Nothing very large to overwhelm the other side.

Declutter your mantel. Only putting about 3 pieces on there.

Take down the large pieces (most of them) that are up above the beam and put some smaller pieces. Add more interest to that area with color and design, but keep it simple.