Our current MBR is close to your first plan, just a bit longer. We have a 5' x 14' master bath with a pocket door between the toilet/shower room and the double vanity, and our ceiling is only 7' 6". We plan to redo the room (it's 1970s era) and remove the wall and pocket door between the "wet" room and vanity because it feels totally cramped. Our entry door is also pocket but is placed more like your second plan, so it limits how we can use the long wall. Have thought about something similar to Plan Two but have almost totally nixed the idea due to the reduction in size of the shower by almost 9 inches from its max potential. Our current shower is 48x30 and it feels cramped to me, which is also attributed to its dropped ceiling - I call it "the coffin." We have no windows in there, and that also makes it feel crowded. We want to do a "tub in shower" idea like the picture someone posted, so we can squeeze a separate tub and shower into the space, but will be compromising by losing the "toilet room" and taking our vanity from 8' to closer to 5'. Our closet is also accessed through the bathroom (door is placed like in Plan One, so we are actually dealing with two doors in our bath - three if you count the existing wet room door), and we have waffled for two years about the necessity of a separate "toilet room" to preserve access for the other person to use the vanity and the closet if someone is in the toilet. Frankly, we have decided that if one is using the toilet, the likelihood of the other trying to use the room at all is very low. We have two other bathrooms in the house and if one of us is using the shower and getting ready, the other would go on to one of the other baths to have privacy to use the toilet, and not depend on a pocket door! That's the reason we think we will nix a pony wall for "privacy" around the toilet, too. Our previous 1950s house had a toilet just next to the vanity and we never used the toilet at the same time as the other person was using the bath (again, we had a half bath as well so could use it if needed) so it's just eating up floor space and visually closing off a tiny room. We are going to have to move our toilet about 3 feet directly adjacent (joists are running in our favor so that is a small consolation) and possibly rerun some H&A ductwork in our dropped shower ceiling (yes, dropped lower than 7'6" - our house was constructed for very short people, I think!). Would love to see what you come up with as we are still in the savings mode to do this project. One other consideration is placing plumbing on the exterior wall if you are in a cold climate. 2/3 of our bath is piped through the exterior wall and I wish it wasn't because we have to be really careful during cold snaps to remember to drip the faucets. You can't "drip" a toilet, though, so I worry!

Solar tube to bring in natural light rather than a window? I don't think there is anywhere else to put the shower that size with that size bathroom. You could also try a clerestory window above the shower/linen closet.

Maybe this could be an option (assuming that since this is a new build, you have the possibility of moving the windows). The closet would need to be 6'10" wide. OR EDIT: The rectangle beside the vanity is the shower, not the tub.

A knee wall/pony wall would be a good compromise. If you prefer to keep floor to ceiling privacy, frosted glass will allow more light into the space. I think the louvered door to the right in your pic is a linen closet. If it is not or you need more storage, a linen cabinet at the end of the vanity will give the tile a clean end. This is how we decided to end our vanity, provide privacy, and add storage, but we didn't tile our backsplash.