I think they look best empty.

Maybe a tall, slender vase or two.

I don't see the point of those niches. Too shallow for much. And they aren't interesting in their own right. It looks like someone as trying to use up some of the left over stone. Weird.

Nice fireplace, though.

I'd fill them in to make the walls flat and paint over it. You'll gain wall space on which to hang something.

Alternatively, I'd fill in the bottom ones to make them flat with the walls, Remove the stone from the top two and put normal wallboard back there and feature one piece of art (painting, photograph, print, etc.) in those two.

I'd be tempted to drywall over all of them. Big art pieces flanking the recess will look great in the end and you won't be restricted in hanging them at the right height.

Wondering if the TV recess should be a deeper tone of the new light paint colour you choose so the TV blends in. Right now the dark wall behind the TV looks good for that reason.

I agree to drywall over these poor attempts of the builder to "decorate". They form huge rectangles on your walls. They're unnecessary. You don't need them to tie in to the fireplace. Your whole space will look so much better without them and when the room is painted a light color. No accent wall.

Drop your TV down at least a foot.

The art pieces you have over the TV would look fabulous in the niches. Do you have any more of that collection?

Another idea, candle holders in different heights. Something substantial.

Is there room in the lower ones to stack firewood? I love that look.

Missed your comment mentioning they have to stay as is.

Maybe it would help you to enter stacked stone wall as a search in Houzz living rooms or family rooms. Decor that is partly organic, larger and fewer pieces. Nothing itty bitty unless they are a worthy collection placed together in one area. The concept of less is more may help you with this room. You mention this area is open to the kitchen so there is more for your eyes to absorb. Your fireplace and niche walls are 'bossy', so let them be bossy.

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If you want to keep them as they are, you could hang or lean artwork in them. Objects, likethese from Restoration Hardware, would be nice too.

I wouldn't say they are pointless. I think they are interesting. Maybe add some recessed lighting?

I think it would help to add some type of shelf too them to give you more options of actually putting something there other than art. I do like the art idea, and adding some type of lighting adds a classy touch!

What about a gooseneck light above both of the upper niches? Would highlight your artwork nicely.

Can't see the right side very well, but I like the size of whatever you have in it compared to the smaller pieces on the left.

I would lower the tv and wipe the console clean of all the knickknacks.

I would do two large matching metal pieces of art on the top two and leave the bottom ones empty. Two silver pieces would really pop and compliment the stone.

I'm an artist and have a lot of square artwork that you can put in the niches - all available in many sizes and colors here: https://www.etsy.com/shop/ShoaGallery

However, I'd leave it alone. The texture of the stone should be treated as "interest" on its own. Also, if you plan to change the colors, do that first, then decide.

By the way, you'd be one of many many homeowners coming to Houzz with the same problem. It's unclear what builders think when they build these features, but here you are.

I think four pictures of matching art could work. Here is one of mine split into four canvases. If you are looking for abstract art feel free to check out my profile. My work is digital so all canvases can be resized.