Almost need to see a pic or two to get an opinion formed for you.

Need a little more info. Pictures would help. Are there windows? Current colors? Open or closed hallway? Straight or curved hallway/stairs? Dimensions? What's your style? What is it that "doesn't look right" to you? There are lots of good ideas here so please don't hesitate to give a little more info to get a lot of feedback.

No to shiplap or any other applied surface added to those niches, which are meant to be clean and uncluttered areas of display. The niches are perfect for large-format artwork -- canvases or framed pieces. I'm sharing a photo of a large canvas painting in my own living space, to show you the approximate size I believe you need for your niches (ok, maybe not quite this large, but definitely not anything too skimpy either). I'm a little envious of your niches, truth be told. Fill them properly, and you'll create a gorgeous gallery in your foyer.

Here are the colors of the walls - The main walls are SW Loggia; the Ceiling of the “rotunda” is Comfort Gray to coordinate with the master bath which is SW Silvermist; I also have Comfort Gray in my Kitchen dining niche to balance the ceiling rotunda. Many have suggested painting the curved wall an accent color - should I paint it the Comfort Gray? Change the ceiling color? Or do I choose another color altogether like Jodie White suggested? The curvature of the wall is 53” wide and 11.5” deep. Again, you have all been so incredibly helpful, many, many thanks.