We are planning a few decorative niches, one at the end of a hallway. I intend to have them done by an ornamental plasterer. I'll admit I'm rather concerned because we have no plans for what to fill them with yet! :) I'm not a fan of them along the wall of a staircase, possibly because I've only ever seen them filled with a vase of fake flowers and I don't love that look. But, this is just my opinion and of course I'm sure they can look lovely if planned well and filled with something unique. I checked out your link and wasn't sure which image you were referring to.

I no longer have my wall niches, but I had them painted two shades darker than the wall & framed the back of the niche with a salvaged iron gate cut into pieces that fit the shape, and then placed greenery in front. My neighbor has 5 niches in her great room wall and has greenery + a coordinating decorative charger on a stand in each. All just different interpretations of "art". :)

I lean toward it doesn't "need" anything, but... If you want something, consider a canvas art cluster. For an example: https://photos3.walmart.com/product-info/1223-3-piece-canvas-cluster-36x-36?theme=wmclusters-1.themepack:splitg3.canvaswrap#|type=3%20piece%20one%20size%20tall These are three 36" tall by 12" canvases, with your picture design printed on them. You can try it out with an online design tool too, and without measurements, I assume they would fit. You might be able to find even thinner. Alternately, if you "want" something, consider some shadow boxes.

Thank you @grapefruit1_ar!! This is precisely the advice I was looking for... I decorate with a variety of Mercury glass Christmas trees, garland, balls, etc for Christmas... so I like decorating this area seasonally... Isn't that the fun of having a fireplace mantel? LOL. I went shopping yesterday and saw some cute wicker vases at Target (too small)... I'm thinking maybe a spring floral arrangement would look good or possibly a birdcage. I've been looking for an antique windowpane or a decorative arched cathedral wood frame that would fit... thinking I could stand it up instead of hanging it ... but it needs to be finished on both sides or I'll have to buy two lol. I have plenty of %&$ to stick up there... but balancing the height in that space seems to be my biggest challenge.