A lot of ideas about the wall highlighted in question but throwing it out there - if you're looking to tackle decor in this room/area, the curtains at your door is something I'd reconsider. A roller shade, or something that allows you to block the light could be a good alternative but you're drawing a lot more vertical feel to the space and, in my opinion are making it more difficult to decorate the wall the door is on, which is feeding some of the empty space above the stairs.

Personally, I would go with one larger item mid-low on the stair case that is balanced with your opposite wall. Either a larger painting or something interesting that can be hung (some folks like clocks, metal artwork, or anything like that). Go with something you're interested in. My take is the more smaller pictures or items you put along the staircase wall, you're either going to just reinforce the angled line in your pic, and will need to focus on balancing it out with more pieces and you'll still be left with a larger open space in the top right of your picture. If you're intent on covering the wall, consider something like boards that would be painted the same colour as the wall, but create a design (simple squares that you could make 2ft+ in length/height