I love art on stair ways and landings, it creates curiosity and a sense of mystery about what is unseen in the rest of the home. Original art is compelling and to my thinking well worth the investment, so I think an oil painting would be great in that spot. With art the best way is to buy what "speaks" to you so if you are uncertain and close to a city with an art museum they often have lend/lease programs where for a relatively small fee you can select an original piece to hang for a month or so. These are not pieces from their collection, but from (usually) local artists. If you like the piece the option to buy is available. It gives you a chance to see what you enjoy. From there you can look to similar prints, if that's what the budget allows, or seek out artists at city art walks, open studio days, or even local art schools. Some savvy interior decorators I know even find great stuff at thrift stores. If you decide to go with a mirror and chest, then adding a beautiful lamp and a large vase with fresh flowers, to reflect in the glass, would be lovely as well.

The frames are too small and too close together. If you want to fill that space with family photos then I would pick three special pictures, get three large black frames, use a large section of white matting around them, and hang them in a row on that wall. I just think the days of the gallery walls of mismatched frames is long over.

Why is the arch cut off by the ceiling? I would keep it mostly bare, but if you need some privacy I would do a simple duette/cellular shade or roller blind that matches the trim color. Place the blinds on the trim between the window and the arch. Done in the same white the shade will mostly disappear when opened. I don't think you have room for an inside mount.

Ah, now I see the pictures. It is a tricky space, but I think you can get the drama you want. So many options will come your way from other readers. A strong tactic would be to choose a rich wall color that you love. Right now you are tending toward a monochrome sepia tone in everything. Years ago, someone told me that a dark space can sometimes be made really dramatic by making it darker, and I've used that advice several times with wonderful results. Over the years I've redeemed dark spaces with rich red, cobalt blue, emerald green, paprika, eggplant purple, deep rose, etc. Every house seems to have a hopelessly dark spot. The first brush-strokes of paint always make me gasp in horror at what I've chosen, but when done and the furnishing put back in place, the effect has always been deeply satisfying. Textiles can be the hardest to find. Perhaps start with a fabric you love for the bench and choose a color from it for the wall. The bench is small, incidental, so don't be shy; choose something that makes your heart sing, and don't worry about it being too this or too that, or what others will think. This is for you! Of course, lighting is important, both for safety in a stairwell and for enhancing the effect. I notice in your sample photo of a house you admire, there are black shades. These are extremely effective in enriching the whole effect, something you can do with chandeliers or table lamps or yes, even on wall sconces. Take your time, picture all sorts of options, and then plunge in. It's mostly just a can of paint and a single day of painting. And, if you don't like it, it is just a single day to paint over it with a new color (which I confess I've done a few times on the way to getting a color right). Best wishes for coming up with something you love, and I hope this message is followed by lots of creative ideas from others. :-)