am curious about the towel ring location. I have a similar layout, and the "obvious" spot seems to be as AJCN suggested, but ti's so near the outlet that I wonder about that. I guess it's not a concern.? In my instance there is a tad more space on either wall. Color wise I have similar flooring and have been strongly considering SW Sea Salt which is similar to watery. I think those colors are nice, but I think the lighter ones considering the dark vanity would work better. In our guest bath which has white flooring and white tiles/tub, I think I really like the Really Teal and the Lagoon. (but we have no windows there just the vanity lighting).

Your deflection rating will need to be assessed. Deflection ratings are CHANGED (increased) by adding PLYWOOD. Cement backer board is NOT structural. It is always advisable to use TWO layers of subfloor. The first layer is attached to the JOISTS (the slats you see are the first layer and are considered 'useless' with tile so you might as well get rid of them). The second layer is then screwed to the FIRST layer (do NOT 'screw and glue'). The purpose for TWO layers of subfloor = the top layer is EASILY removed if something changes/goes wrong (glue causes issues with removal = damage to bottom layer = more problems then it is worth). Right. So now you know you need TWO layers of plywood. You can get two layers in there with a total of 1.5" (3/4" + 3/4") worth of 'thickness'. Now you have the RIGHT deflection rating (super important) and you have added quite a bit of height. Now you assess how much extra height you need. A well installed porcelain tile can sit (roughly) 3/4" high. You can add cement board to make up the little bit of difference. There you have it. Two layers of plywood + cement backer board to make up the difference.

I would deepen the shower where it stands. Use the little bump-out in the tub space by building a 4 ft high wall even with the regular wall and then be able to use that new wall depth to put in a ledge (in lieu of a niche) at the 4 ft height and a toe-rest for shaving indented into the new wall at the proper toe-ret height. To get storage, get a trough sink with two faucets, leave plenty of counter space on either end, and use 2 ft of your enormous vanity to build a linen closet. If you think you need more places to put things down while getting ready, put a ledge just above the faucets. I will go look for a photo to show you what I mean. Holy cow! The iPad app now lets me make links, just like the regular computer app always has. When did that happen?

Smaller windows above the water line,and a big shower. I have a 4 1/2 x 10 shower and I thought it was ridiculous (and to some it is) but I really like having my own area. .We have shower heads on each end with a rain head in the middle. If anything were to happen, it easily would accomodate a shower chair or an extra person if need be. We are in Florida so temperature and being chilly isnt a concern. I wouldnt dissuade you from building a big shower. You have to weigh whats important to you. I had the same size shower as you have in my last home, the larger shower is a game changer.