I'd use at least part of the space to build storage like a pull out pantry and set it on long drawer slides. Or a couple of them stacked. Between my toilet and my vanity cabinets I built a dividing cabinet. The dividing cabinet carcass is about 48" tall, maybe 12" wide, and about 30" deep. That cabinet carcass holds a small pullout drawer up top and a larger pull out "pantry" in the lower portion. The lower pullout pantry is on 28" slides and it's divided in two with a vertical partition, and on each side of the vertical partition I have a few shelves. When the pantry is pulled out one half faces the sink vanity and the other half faces the toilet. The part that faces the sink is used by my wife almost like a medicine cabinet. The shelves are only about 4" deep, so it suits that purpose just fine. The shelves that face the toilet are deeper, about 5", deep enough to hold rolls of toilet paper, paper towels, cleaning supplies, etc. Since you have a 45-degree wall to the left that would hinder access, I'd just make a unit that opens to the right. You could get 10" deep shelves for decent storage. I'd say to rip and replace the walls, but I see the shower valve is on that wall and you already have a niche in the shower on the other wall. Regardless, be careful with the plumbing since it's a wet wall. The room on the left is laundry? Can you access any of the space from the laundry room for wall storage on the upper parts of the wall? Use the lower part of the cavern for a pullout pantry accessible from the bathroom, use the upper part as in-between stud storage accessible from the laundry room.

If possible, paint it the same as the wall color. Then put a console table under it and either a mirror on the door or artwork....taking into consideration of the latch....with something hung within the frame, you probably won't notice the latch. If you have a plug in, put a lamp on the right side of the console and something low in front so that the door can be opened fast! If no plug in then put some pretty candle sticks on the right side and something low in front. Judging from the door knob on your door, this opening is quite high....maybe a taller chest of drawers would be better, but am thinking a console table would be best. Or you could go lower and put a pretty bench in front of the opening after you have painted the opening the same color as the wall.

I am of the school that sometimes blank spaces are awesome. Not every nook and cranny needs to be filled. Pottery needs to be taken down every 6 months and cleaned and dusted. You could always have cabinet toppers built with glass doors and lighting.

To me, the point of keeping it would be because it’s original to the house ... so I wouldn’t paint it or modify it. If that were the case (not happy with present looks), then I’d just remove it. But it looks like you decorate in this style, so I’d embrace it as is. :) I know you said something non-plants, but there are very realistic faux plants, if you change your mind. And I’d use smooth black stones or colorful pebbles. Just quick examples, there are so many places that sell faux plants — https://www.afloral.com/products/mother-in-laws-tongue-in-variegated-green-24-tall?gclid=EAIaIQobChMIh5yYxKPP6gIVhYbACh2ekwygEAQYAiABEgJYIvD_BwE https://www.afloral.com/collections/silk-flowers-silk-plants-greenery-leaves-grasses/products/faux-fern-leaf-bunch-22 https://www.afloral.com/collections/silk-flowers-silk-plants-greenery-leaves-grasses/products/artificial-eucalyptus-spray-in-green-and-grey-pse331 As the unit is very closed-in with the door, and has lots of hard elements around, I think plants would soften the area, and provide nice interest and texture from both sides ... as well as have a simple and organic feel that would mesh well with whatever decor you use on the shelves.