Love the plant idea!

I think you might be able to fit a tall plant stand there, with either a plant on top or perhaps a decorative vase. You may even have space above it for a long(ish), narrow framed print.

If you're ambitious and need storage, I could see a custom made bookshelf with angled shelves.

Or something like this:

I like to go big. I think it is a great space for a tall floor vase with tall filler.

It would have to be a silk plant if you want it to live...

Sadly, no to a mirror on the narrow wall seen from the living room, which would add both light and feeling of larger space. It would also tend to reflect from some points in the living room the bathroom door when closed, the bathroom when open, and people entering and leaving. :)

At the moment that's the only contribution I have. I feel as if you do need something on the part seen from the living room, not just the other. So the hall's not obviously utilitarian with nothing to note but doors. Definitely nothing that would draw attention but rather quietly achieve an attractive harmony with the living room decor that will satisfy a roaming gaze and allow it to pass by smoothly to other areas. Very interesting problem, creativity time.

Can't tell from your pics what else is going on decor-wise, but I'd be tempted to do something whimsical.

I also like the plant stand idea, if you can find a plant that doesn't need much light.

CB2 has a few plant-stand-esque tables that are kind of cool looking. You could use the taller version of this and put a plant, or a vase of flowers, or something else you like, on top.

https://www.cb2.com/marble-rose-gold-pedestal-tables/f11828

Do you need a place to hang people coats? You could add hooks there. Very odd little space.

I would just do a long piece of art down the middle or a very shallow tall lingerie type chest.

A tall, narrow tapestry could work there. Or, three stacked pictures hung one over the other vertically, under the section that has the smoke alarm on it. The photo makes it look like a narrow walkway so that anything that takes up much 3 dimensional space might get bumped or feel claustrophobic. Is it bigger in real life than it looks in the picture?

If the smoke detector could be moved, and this were my place, I'd be tempted to add shelves and a pair of bifold doors, turning it into a linen closet.

I would put a full-length mirror on the angled wall, but this might be bad feng shui. However, according to feng shui, a mirror should never reflect another mirror, and I used to have this in my bedroom, so that I could see myself from the back, and so I'm not sure how to take some of those rules. I would avoid putting anything that might appear like clutter in that corner. I might just put a coat hook there.

If I had that space it would be full of bookshelves and art in about 30 milliseconds

Do you have enough storage areas? If not I would put it to practical use. It would take very little time and money to make it into a shallow storage closet. Or, an angled mail center, put a cool artsy coat rack there or a built in book case.

Or, do a removable corner plus wall wallpaper mural. This is a cool site. You can even upload your own picture to be made into a mural. By wrapping the whole area in a mural it will not only look good but will help diminish the angle.

https://www.eazywallz.com/collections/living-room-wall-murals

I think I would put three pictures one or top of another there and call it good. Sometimes less is more.

justerilynn where did you find that rack in picture number 3? I LOVE it!

I'd have a cabinet maker come in and do a custom piece for it. The angle looks too weird imo and anything put there would accentuate that. But I nice cabinet built for storage would finish it off nicely. I see there is a sprinkler system on the ceiling. You could have space at the top to display something and have it lit up.