I saw a photo that a houzzer posted where they had cut out the area beneath the stairwell & added a door for storage. It was just posted a day or so ago. See if you can find it if interested.

They cut a standard door in half & were going to add Oil rubbed bronze hinges & a glass doorknob. I think it would be better to underplay it, but how great for additional storage. I've also seen staggered bookshelves added under stairs too.

More photos from different angles, please. We have no idea how much space you have, where the corridor leads, what it looks like when facing the stairs. Can't advise from this one little glimpse.

Here's the thread Sandra mentioned above: [https://www.houzz.com/discussions/does-this-door-look-weird-dsvw-vd~3635770[(https://www.houzz.com/discussions/does-this-door-look-weird-dsvw-vd~3635770)