I like books and baskets; here is how I did a case for one client. Dont over do it. Start slow

https://www.houzz.com/photos/joni-koenig-interiors-traditional-los-angeles-phvw-vp~877550

Here is another example; Keep color to a minimum

https://www.houzz.com/photos/urban-traditional-transitional-los-angeles-phvw-vp~25879686

Hi Kat I would like to see your art moved out of the niches & onto the walls. The art will have greater impact there & the mantle area already has a lot of rigid vertical lines going on. I think some sizeable sculpture or artglass (or whatever you fancy) should be placed in the niches. These items have fluid lines to break up all the vertical lines. I don't think you need anything around the TV. Make the pieces in the niches the stars. A very rough mockup to illustrate what I mean

Thank you both for the feedback. I'm not sure where to incorporate books. Maybe stacked at the bottom of the niches? But the niches are only 6" deep. Would also like to keep the artwork in the niches because I love them and this room is open on the other sides so there is no other wall space in the room. We looked into putting decorative objects in the niches but it was difficult to find large pieces that were also narrow in depth.

Our main floor is open concept and this wall is the focal point from our kitchen and coming down the stairs, so I really want to get this right. It's been difficult finding photos of similar built-ins - most are bookshelves above the cabinets.

I like that. Would need to find some houseplants that are non-toxic for my cats :) Do you think I should put anything on the mantle with that arrangement, or would that be too busy? It's 15" deep on the mantle so there is a pretty large space for picture frames or something else.

I like the decor as you have it now. I wouldn't put anything on the mantel by the TV. There's a lot of molding detail on this wall, so I would prefer to see it painted all the same color.

Hi, the detail on your wall is beautiful. I'd paint everything to match the FP panel. This includes the cabinets. Then for some punch to the arched niches, add either antique mirror, or, a greyed-out metallic wall paper for it's shimmer. In front of each niche on the stone surface, place a simple off-white ceramic container. Into the containers use a pair of plants like palms, orchids, or fern. The tops of the plants should go no higher than 5"-6" below the arches. Use your artwork on another wall. That's it! I'd keep the mantle clean of accessories.

I would not put anything on the mantle - there is a lot going on there - TV, accent colour plus moulding detail. Anymore and you will detract from the elegant look you have achieved to date. If you decide to go with something similar to my last mock up make sure that there is a little overlap in height between plant height & niche & candle holders & niche that way all parts are connected. I personally think plants add a softness to the area.