It can either look really dramatic or too dark, depending on the size of the room as well as your other components. I've attached a couple of images of our work using dark tiles, the smaller bathroom we have surrounded the bath with dark tiles but then balance it with an extra large mirror to bounce the light around the room. The larger bathroom we have taken the floor colour way onto the wall but used a different texture on the walls for drama. Hopefully this gives you more of an idea but you shouldn't to nervous of the room being dark, it's where you can create a contrast as well as textures. Look forward to seeing what you go with in the end.

Hi bigreader. It's not the tiles in the niche that are the problem. In fact, one piece of tile (30 x 60) will fit perfectly with no grout lines. It's the wall tiles around the niche space that will not line up with the side edges of the niche

One long shelf rather than actual 'niches'. We usually build another 90mm thick wall right in front of the other, then linings and tiles, etc. Cheaper and looks amazing but it will narrow the room by 90mm which may or may not be ideal. One of my clients is going with a 120 mm wide shelf in their large ensuite! It'll be a shelf but also hide toilets hidden cistern too.

Look at what shampoo bottles you use and size accordingly. I know we have 2 pump bottles pls bath liquid and a couple of extra it,es for individual needs.