We suggest you break up the monochromatic color scheme. Bring in some light blues, slate grays, with a hint of green. We'd bring in light blue/ silver towels as well. I can't tell colors exactly without color matching, but consider gray/sage as well.

For art -think abstracts, or a landscape. You have a lot of repeating pattern in this bathroom - I suggest breaking it up and keeping things airy.

Are the floors heated?

I would not go for an outdoor rug. Yes, they are waterproof, but they are also not too much fun on your feet, and in a master bath, our clients like to feel pampered. Once you figure out the color scheme, shop sales to get yourself an appropriately sized plush rug, and figure you may have to replace in in a few years. You shouldn't have to, unless it is too big, but I always tell my clients to think of that.

Good luck!