How deep is it? If not deep, maybe an interesting textile sculpture. something that could be bent to follow the lines of the wall.

It’s hard to tell if there’s room to set a thin sculpture there. Or hang a mobile-type art piece?

Goodness - I'm usually quite fond of niches, but whomever installed this one did you quite a disservice! It really doesn't appear to be deep enough for anything other than a textile, as has been suggested above. That said, the right textile (something large) could make a gorgeous statement here. Do you have anything of a fabric-art nature that you could audition here?

- well I'm glad you all feel my pain! :) it is quite narrow (shelf 5", actual cutout 4")

if you do some kind of fabric sculpture or textile, it could project a bit into the room, much like if you hung it on a flat wall. There are some amazing felt artists out there doing gorgeous stuff. Not sure where you live but I'm sure you could find one in your area or online (etsy maybe?) and purchase something or have something commissioned. Look how beautiful this is!

Get a tall, thin glass vase and put in some twiggy floral arrangement. Add a light at the top to highlight it.

Modern Elegance Contemporary Living Room - San Clemente · More Info

Alternatively, get a contractor to re-frame it with several openings to make it more of a statement upon entry. It doesn't have to be curved. Something like these:

Native American Pots Display · More Info

Curious - is this a new built home or an existing home you moved in to? Curious about what was there before. Personally I see a quilted work of art wall hanging. Either that or a custom metal hanging made to fit the curvature and have the contrasting color of niche show through it to highlight. Neither option will be inexpensive but if it’s your long time place to hang your hat, well worth the years of enjoyment you would have!

Find a great wallpaper and wallpaper it.

You could maybe build a custom shelf to protrude to make a better space where you could set a vase etc.

My office building has a huge round ground floor seating area with an enormous painting on the curved wall. The painting consists of a patchwork of canvases, unframed/unstretched, that follows the curve of the wall. I'm not sure how they are mounted (the wall is marble), but assuming your niche is drywall I think you could probably find a way to attach an unstretched canvas, tapestry, or textile art to the wall (staples? nails? You could sew the artwork to sturdy backing fabric for the actual fastenings, to protect the artwork).