I would start by hanging a light fixture to shine in the window and thenchoose so less generic art that you love for the space but really that is an odd place to hang art

stand on the landing and face the wall mark a spot at eye level... 55" - 65" off the floor for many. IF you can center a piece of art at the eye level mark THEN you have room for something, otherwise, you don't need anything.

I would only hang something narrower than the window opening. How about some kind of window covering hung from the wall near the ceiling? not to mention a light fixture.

If you need some color a rug on the landing? or a shelf at 48" off the floor?

I would not add anything to that
wall. The window is your accessory. Keep it simple.

I'd trim off the window with some beautiful moulding, and if there is enough room put a narrow sitting bench or console table underneath. If no room for that, a tallish plant in a really nice pot in the corner. But definitely trim the window.

That triptych feels heavy under the bright window, so I'd either leave the landing wall quiet or swap in one calm horizontal piece with a lighter, modern palette, kept narrower than the window. I wouldn't add a bench unless the landing has generous clear space; keeping the stair path open matters more than filling the wall. Simple trim around the window could make it feel finished without adding clutter.

Agree with just trim around the window.

There is an over abundance of 90 degree angles in this staircase. Maybe something more organic would provide some relief. Something 3D would help.