No tub? If not, then there are lots of things you can do. Do you have a sketch that shows where the doors and windows are located?

I think a very large niche...with arched top like your wall niche picture or not...could look great along that wall. BUT...only if you do something to make it stand out. I'm thinking of something like a mosaic or different tile motif similar to what folks do behind the range in the kitchen. If it is simply tiled in white subways like the rest of the shower, I fear it will also look, as you say your current shelf does, like an afterthought. But if you put some other tile in there, it could look WOW! Don't know what the rest of your bathroom plans are, so I hesitate to suggesst something specific. But perhaps picking up the tone and texture of the tumbled travertine on the floor? At the very least, I would suggest tiling the shelf of the niche in the floor tile. I'll see if I can find some pics too.

Mix of metals doesn't seem worth worrying about -- that's pretty common these days. I might turn the light fixture to face down, but just because it seems unnaturally high. For the corner/mirror, I think I'd try to find a nice wall cabinet that could go in that space, to act as a linen closet and give you a bit of a more intentional-looking solution. It's probably ok to block that wall switch if it's a duplicate of controls you have elsewhere, but you have to stay slim enough to avoid the window. Ikea has the right kind of thing. (Most of their explicitly bathroom cabinets are tall and narrow, but you could use one of the other bookcase/door/basket systems to the same end.) Otherwise, you can't do much to make it less brown, other than by putting in some colorful rugs and towels with art to match. It's possible that the paint could be a bit cooler, and I suppose you could paint the vanity from red-brown to white, with some effort. But really, this is a perfectly nice bathroom that just isn't to your style. Dress it up in things that feel more like you and then don't worry about it for a while.

Sativa has a good idea for a high contrast design but I might be overwhelmed too much dark black. It would need the balance of lighter floors and the natural light from the windows which could make it stunning. I bought the same pebble tile at our local H H Restore to re do our tiny shower. I want bright white tile like your subway tile but hubby still wants a large format porcelain. The mix from the pebbles can work with many things. Remember when it is sealed the colors will deepen to what it looks like wet. You could use a midtone like the charcoal grout for your floor tile if you are keeping the walls lighter like a warm white, tape or grey from the pebbles. If you go with dramatic black walls, you need a lighter grey ,taupe or warm white for balance. With either color scheme choose a basic floor tile with a bit of texture to prevent falls. A rectified porcelain with touches both cool grey and warm whites and taupes would work with lots color schemes. Keep in mind you may change your style in a few years. So pick a classic with lots of options. It will be easier to change a dramatic paint color than a busy tile pattern.