Lean a rectangular piece of art against it. The top of the art should be even with where the arch just starts to happen.

Tall narrow plants in front of each window will add life and great color to all those neutral colors.

I love the Blue and Yellow you have touch of in your dining room and kitchen. Continuing those in your living room and over the fire place would tie the rooms together.

The brick above and below the mantel doesn’t seem to match. Is it just the lighting? If they don’t match, would consider painting the brick or doing a wash?
If you really want a mirror inside the recessed arch, I would have one cut to fit the space. If you would like some art there, I would select a rectangular piece as wide as the edges of the arch and as tall or taller than the arch.
Every room can benefit from art on the walls (any kind of art).
And every room needs a bit of color.

Just cover it with artwork.

You have 2 half arch and 2 full arch windows already in that small room. That's a lot of arches. I would hide the brick arch with a rectangular mirror and replace the fire screen with a rectangular one. I think the half arch windows look like they are floating. There needs to be something with color, maybe in blue, underneath them (same thing on both sides).

Well....I’m not a professional, but I can tell you in all honesty that I would wall up those windows....in a heartbeat. There’s just something incongruous about them that I can’t get past...like they’re little eyes looking at you! If they were larger...like 3/4 of the way to the ground...think they would look OK...but those just look like an afterthought. The mantle is very nice...needs substantial decor...big, eye catching.
Maybe Grover can do you a photoshop that removes the windows.

Maybe this is what needs to happen with the windows.

That mirror gives a great view. Of...The...Ceiling.

Clearly the wrong place for a mirror.

Thank you Groveraxle and beverlyfladeziner for the great ideas!

sorry for the hijack....@BeverlyFLADeviner, where is that artwork you recommended from?

Do not like the round metal object and assorted ceramic vases. They just emphasize the arch. Get a large rectangular painting that covers the entire arch. Either take out the windows or put lovely plants in front of them.

karrilouwho, I've seen that canvas print on Ballard Design's site. I believe it's titled "Southern Oaks."

Nice suggestions from grover and Beverly! No mirror there, for sure.