If I am understanding your question, that space doesn't need anything. The niche next to the fireplace needs something.

I have marked the space in this picture. You suggesting not to do anything on the top and put decor on the bottom one?

Maybe ignore the above space and just decorate the lower space to match your drapes.

Do you need to keep those curtains shut? What is the view?

If this were my space I would replace those drapes with something that blends into the space so not to add color to the above. And if the view were acceptable they would be open all the time. That blue stripe only draws the eye up and you see the empty space. I would have drapes walls trim all the same color.

Here are some ideas...

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Paint the interior of all the built ins and art nook like this... Then a vase or sculpture will come forward.

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Thank you! These ideas are really helpful .

I was thinking maybe a huge plant?

Also, I have similar space on the other side as well.

A heavy statue/something heavy could be dusted from the floor with an extension duster. It wouldn't need it weekly as no one will see the dust way up there and no one is touching it. Cob webs will be the thing to watch for. A fake plant would be problematic for dusting from a far. This home was evidently individualized for someone with a plan/a collection of some sort. Bummer for you

You might consider doing a statue in the niche up top, but a tall tree (or plant) can work well in the empty niche below it. Please see our two designs below for statue and plant ideas.

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Two metal herons.

Put nothing there. If you don’t want to close it up try some of the remote mood lights that change color, however - space will eventually need to be dusted, remove cobwebs, etc. Anything heavy or fragile can risk getting knocked down when dusting with a tall wand duster and who wants to climb up the ladder to change decor. I had a similar space in my last house, and put some frosted glass square lamps but they could be turned off/on from wall switch. Annoying to have to get tall ladder out to replace a light bulb and the clean off shelf because dust could not be ignored.

An idea for your space.

I totally agree with kandrews. It's not a very functional niche, and just because it's there, I wouldn't want to draw attention to it.

I will also note that you must not live in California, because no one in earthquake country would consider putting a piece of ceramic or a potted plant up high like that, especially over a seating area. I suppose you could bolt it down.

I think I would address the lower niche first. I would probably go bonkers leaving the upper one empty since it is there after all including a light. Perhaps something to consider would be to have a very subtle mural painted in that upper niche, something that strikes a balance with the aesthetics of the entire space in that area.

Do the hard thing and have the upper niche walled over. The bottom will be on you to get your clients needs met. Dry bar? Book shelf? TV out of the fireplace ?

I agree with arcy--close the upper niche with drywall. It serves no purpose and is just a nuisance.

You could then place bookcases or shelves in the lower niche. Or console table with artwork or a mirror over it.

The niche up top breaks up the expanse of wall. It's not terrible. Just don't over decorate.

Not every expanse of space needs to be filled.