Pull out Wardrobe Rail for under stairs. You could have a few of these attached under the higher stairs and some storage boxes or even a chest of drawers underneath, then you could do away with the black frame and the room would look / be much larger.
DIY: Pull-Out Storage Case, use existing bookcase long drill bit to feed axel thru wood or reinforce bottom with another board, could this work for pc stuff, cleaning supplies or shoes, what about lining up/stacking a row of shelves or shoes? cover up with curtain? or inside a closet on the side against the wall, what about short bookcases, in the bathroom closet or under the sinks, love this!