I am in process of designing a shower with the same dimensions and questions. Same drain plan. Preliminary decision is to go with Kohler's hydrorail shower system which is exposed, includes the handheld diverter and supports a rain shower head on the 18" extension. That puts someone showering between 18-24" from the shower head wall. It leaves considerable space in back for a bench or a chair. The advantage to me is simplicity -- that system supports a single pressure balance valve and has an integral diverter. Alternately, to do a ceiling mounted rain head plus a hand held, a thermostatic valve is needed and the price escalates. For a master, I'd likely go with thermostatic. We have one in our master with a separate hand shower and it's excellent. But for this powder room bath expansion I think a single pressure balance (on/off) shower is sufficient. It's really a question of how the shower will be designed/used. I don't have experience with the hydrorail so I did do a post asking if anyone had used it.

I’d like to see any waterproofing details. Which don’t exist, as someone just plopped lumber down over an existing floor to create this shower. And then installed done rubber. with no preslope. It hasn’t even begun, and it’s failed. Badly failed. Expensively badly failed.

This is a bit off topic but the space between the vanity and the shower looks tight along the path to the walk-in closet. The gas chamber looks confining (think public toilet stall), especially with a tall ceiling. Because of that and the shower head arrangement, it leads me to believe there are other areas in the house that could use some adjustment. Has the construction started on the house? If not you may want to post the floor plans and exterior elevations and site plan to get some review. But as always . . . WARNING: Posting your entire floor plan opens you up to facts you may not want to know, but should. Be prepared for suggestions that will make for a better design. Keep a thick skin, open mind, sense of humor, and a glass of wine within arms reach. You may not enjoy it, but everyone else will and you will be better off in the long run if you heed well the advice.

I would try to stick with a 60" tub in your shoes to allow more space to enter the tub, even though I know that will cost you vanity space. Alternatively, can you do a wall-hung toilet, or is that not feasible with the building restrictions? That would also give you some additional clearance. We have a layout like this with a full-height partition wall at the end of the tub, and all the plumbing, shower head, and controls are in that wall. It works well. We always turn it on from outside the tub and then step in once it's running and warm. At some point in the past, the plumbing was clearly on the window end of the tub [old drain and pipe holes in the subfloor], but that puts you right up against the toilet when you're turning it on so I can see why past owners moved them! (I assume before they put in the current setup--which we think was done circa 1955--there was no wall at the end of the tub and it was either clawfoot or rounded and open, so the plumbing had to be at the opposite end.)