I can’t tell how much space there is between the wall and the staircase. What is here, where the arrow is pointing? I assume it’s a dead space with no reason for a person to walk there ……

Add a photo so we can see the area straight on between the wall and the staircase that @littlebug Zone 5 Missouri has pointed out. If that is a walkway, how wide is it?

Another option is just one large piece of art for that wall.

Yes dead space only about 16” wide

It’s hard to find a picture of a dead space beside a staircase, but I finally found one. Instead of more decor under your art, have you thought of putting something inside the dead space?

Select a Chinese entry cabinet in a Black finish.

I would love to find a cheaper version. I love this look.

https://www.overstock.com/Home-Garden/SAFAVIEH

All inexpensive from Safavieh.