I would go with a big bold piece of art. Like the ottoman in this picture, too.

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way cool stairs.

You might consider making the wall visible beneath the stairs an accent wall with either a different paint color and/or wood paneling (or cabinetry) to "warm" the spot; and/or then

Make use of it for a specific purpose (depending upon what is adjacent to that space. Some kind of cabinetry that meet a specific need would likely be your better choice.

If your family has items that reflect personal hobbies and interest, displaying them -- or family treasures (framed photographs, collectibles, mementos, trophies, awards, etc., here -- whether in open shelving and/or behind the glass doors of a curio cabinet, barrister's bookcase or even a recycled china cabinet -- could be worth considering.

If you have an indoor pet, making that spot their spot is worth considering.

Not sure I'd want seating beneath open stairs without an overhead cover.