It's not that hard to find narrower vanities (19-20") and that might help. I agree with weedyacres, maybe consider a pocket door for the bathroom door.... 31" clear for the toilet is just barely enough IMHO, next to a vanity. A couple more inches would feel much better. As far as tub or shower... how do you envision using it? For example, would it be the place to wash dogs or really dirty kids or soak houseplants? In that case, a tub might be more useful. if it's just an overflow, occasional use shower (or for resale), then a small shower would be fine. I am not sure it'd be necessary to go with what most people consider the minimum comfy shower size (3 x 4) if you'll rarely use it and you're trying to maximize the space... I guess what I am saying is: what are the priorities for this space?

Thanks much and beautiful showers! I am confused about the thermostatic valve. Is this something that I can buy separate and have it installed in the wall so that anytime I change out the shower handle/trim/valve it will automatically be there and still be accessible if it breaks and I need to change it? I hate to have more moving parts and things to break inside the walls. LIke this (although it looks like part of this would stick out of the wall?-another hole?: https://www.amazon.com/Rozin-Thermostatic-Mixing-Temperature-Control/dp/B01FAGWKTC/ref=sr_1_3?ie=UTF8&qid=1542006430&sr=8-3&keywords=thermostatic+control+valve#customerReviews  Or does do certain valves come with it like this?: https://www.amazon.com/Hansgrohe-04231000-Thermostatic-Control-Diverter/dp/B07FDFK5SM/ref=sr_1_1_sspa?ie=UTF8&qid=1542006430&sr=8-1-spons&keywords=thermostatic+control+valve&psc=1 I was wondering, also, it sets the temp but the initial issue of being cold while the water warms up since it has to run through the pipes would still be there, right? Problem is, I have chosen champagne bronze and if I need to get the thermostatic control valve built in I think that will be hard. I'm already having a hard time finding a handheld and shower head built into the same. I might have to get a separate handheld to divert water or I'm back to the initial question of do I just push the shower head over to shoot towards the wall before I turn on the H20 or go back to the idea of moving the control valve? Here's one of the options I was looking at for a showerhead before this discuss that I might need to adjust: https://www.amazon.com/58471-CZ-PK-H2Okinetic-4-Setting-Handshower-Showerhead/dp/B009MLXJN8/ref=sr_1_2?ie=UTF8&qid=1542006748&sr=8-2&keywords=shower+head+combo+with+handheld+champagne+bronze

Our shower is 6 x 4. We chose to install 2 valves. Valve 1 has a fixed showerhead and a rain shower on a 3-way diverter. Valve 2 has a handshower positioned near the teak corner bench. For a true dual shower you might want to install 2 fixed showerheads on opposite ends, and one central rain shower. For ease of cleaning, you might want to change one of the fixed showerheads to an adjustable handshower on a slide bar. It will still work the same as a fixed showerhead, but you could use it to rinse the shower down after cleaning. If you post a drawing of the shape of your shower, where the door will be, etc, etc. people can weigh in more.

For two shower heads and this configuration, you'll need to put a full wall next to the toilet so you can have the shower heads across from each other. The only area that closes in is the toilet, so I think it's fine. You'll still have the long side of the shower as glass, with the window providing light to the vanity, shower, and tub where you need it.