Personally, I wouldn't hang anything shiny like tile or metal because when the sun hits that wall the glare could be blinding. Instead, I would want to soften it up with bamboo in planter boxes, of plants in the new wall containers: [houzz=]

How much do you want eyes to pop? Would you consider getting a mural painted on the top section of the fireplace? Not an inexpensive thing to do, but you would have a one-off. As attractive as stone etc might be, it's not exactly an eye-popper these days. I'm ignoring my wilder colour imaginings because your room looks very gentle and pastel. If you can show the new paint, a picture of the rest of the room could also be useful.

We retailed the fireplace in our bedroom. We had a similar tile to yours and wanted some more color. We ending up pulling the old tile and patching the cement to be ell and tiled and grout. I would imagine you could do the same

You already have a bunch of shapes in the room, round window tone, lines on the hardwood floors ,squats on the ceiling, bricklayer pattern on the fireplace, I think the diagonal will yes be another shape to the eye but in a good way by not adding another of something already in the space