HI -- I think you need to have wood panel done on the back wall of each side made to match the rest of the wood so the spaces blend in more with the rest . Maybe have some smoke antique mirror in a grid pattern for the center section . Or a rich tone on tone or hand painted wallpaper would be interesting . It would have to be grand to go with the grand feeling of your room. Then maybe just display one big thing on each side . Also go to some of the big fancy hotels to see their ideas for big displays or in high end department stores. Or just hire an interior designer to help with this one project . Your wall and the house look beautiful and massive !

How about a slightly amended version of your wallpaper idea? Choose a soft, pretty, wall paper with a watercolor-y, blended, not too much detail design, in muted tones of your room's color scheme... then, and I don't know how to accomplish this, adhere it to the 3 big empty spaces above AND the back of each shelf below.... keeping the pattern as if it had been installed in one large sheet behind all of the wood. Another way of stating it, install the wallpaper as if it were on the same "layer" as the fireplace, the "layer" on top being the woodwork. It would be busy so you'd have to go simple on the shelves. 1 or 2 simple plant containers with green trailing plants & companions... same on both sides of fireplace?

I think that the wallpaper idea is ok, but slightly overdone. The built-ins are elegant as they are. I would do large matching mirrors on either side. Something round to break up the lines. In the shelves, I would also eliminate some of the small items, and add some large hardcover books that are stacked, mixed with a few upright. Though I am not an accent wall fan, painting the large arched area over the fireplace, in a dark color with a great piece of art would help as well. Good luck!

Another thought, beautiful cabinetry is so tall, something VERY low on tops and bring eye down with plants or an uneven number of items, large, med, not too small, placed on one or both ends of hearth. Or one group of items just to one-side of the actual fireplace. OH... beautiful mirror over fireplace, don't fill space to top tho' and SLIGHTLY tip mirror down to reflect your room.

I agree with L. Huck -- leave the large spaces empty. There's a lot going on along that wall, so adding more along the tops would be overkill. I would also replace your branch with colourful framed art over the fireplace. Good luck!

This is QUITE a challenge!

Cabinets need oversized decor to have impact. Style shelves with 1/3 books, 1/3 decor items, 1/3 open space. Do you want to draw attention to the space above or minimize it? Some super large art with distinctive subject matter could make the space.

What was your final solution for the, somewhat awkwardly designed, built-ins surrounding your fireplace?