Yes, the "dead space" is a hallway. I will try an mock up a better rendering tonight. The space is a bit convoluted. For now, maybe this picture helps? If you can tell below, the blue wall space next to the doorway in front of the left side open shelving is the backside of the bathroom. This is not load bearing. What I'd like to do is drywall over that doorway, and move the bathroom door forward, adding about three and a half more feet of space into the bathroom from what is currently just a hallway that leads to the bathroom and garage door. The hallway runs the length of the backside of the kitchen run where my fridge and oven are. If you are walking down that hallway, it ends with the bathroom door straight ahead, my garage door to the left, and that open doorway into the kitchen on the right. That photo is captured above in an earlier post. So, from the hallway, the left wall has a door to my basement, the a closet then runs along my garage and the right side is opposite my kitchen. We have a full basement, not crawl space. Moving doors should not be a problem.

I wouldn't use beadboard because it's cottage/beachy and your bathroom is traditional. If you want the beadboard look, I'd use a Shaker vanity. Otherwise I'd use traditional paneling with that vanity. Here are a few examples: With a warm color palette, you could use polished nickel for a little sparkle. It has a gold undertone where chrome has a blue undertone. The vanity is heavy and ornate so I'd probably look for a wood framed mirror for balance, though I'd keep it simple. I like rectangular mirrors when you are using an overhead light fixture. Something like this: https://www.build.com/avanity-brentwood-m24/s899881 Or the Avanity Madison: https://www.build.com/avanity-madison-m24/s1085061 The floor, vanity, and mirror hits my limit on brown tones, so that's why I'd use nickel lighting, faucet, and hardware. I'd use a traditional three-light fixture that's a few inches shorter than the mirror. What will the countertop be?

I love the art AuntThelma posted, too. It gives your dark walls a nod, while also bringing a splash of color. It brings a restful feel to the space above the resting bench and reflects the scene out the window. Your tall furniture piece is lovely, but it could only go in the space between the tub and the pony wall, right? If you need it for storage, then you should fit it in, but I like the feel of the room without it. I happen to have a few teal things saved. Though their ocean theme does not go with the picture, they might be useful if you did use the tall furniture piece. This octopus just makes me smile!

Do you have an elevation drawing of that stairwell in the living area? Seems the space under the stair landings could be better incorporated in your effort to locate the powder room, or at least the hot water, accessing from the closet by the entry and moving the sloset to the living room side. On another subject, am I mis-reading or are there bi-fold doors by the tub in the main bath? Looks like there are windows in the WIC. I would rather have windows in the bath and walk through the closet to get there than have to walk through the bath to retrieve clothing; not convenient if there is someone using the bath. That pantry at the foot of the stairs looks like a good spot for a stacking WD. I'd want it configured to vent out an exterior wall, using pullout pantries in the kitchen so that space could be the laundry. But first I'd move that big bath's footprint to the deck end of the space so the closet could be in the master.