There are pocket doors that will work in a 2x4 wall, though for new construction a 2x6 would be preferable. I personally can't stand barn doors but some people like them. They are definitely less expensive than pocket doors, but in the grand scheme of building a house, not sure it would make a huge difference. Do you absolutely need access from the guest bedroom? If you have infrequent guests and they can easily get to the hall to access from there, I'd much prefer that. As a guest I'd be super nervous about someone coming in from the hall direction (one reason I don't love jack-and-jills in general). I wouldn't have an expectation of an en suite bath as a guest, and this isn't exactly that anyway. But that all depends on how close the hallway door is to the bedroom door.

So I guess the short version is: if the bedroom door isn't too far from the hall door to the bath, I'd just close the wall up and forget the door from the bedroom, barn or otherwise. If you will be using the bedroom for long-term guests (weeks or months at a time vs. days at a time) I might feel differently, though.

And pocket doors do not offer much more privacy, sounds still travel easily around the door.

A barn door will eat up what might be valuable wall space in the guest room. I’d go for the pocket door for sure. I & my husband were guests for an extended stay in a home with this same set up - pocket door into guest room, swing door into hallway. It was perfectly fine. We simply locked the hallway door at night. The guest room & bath were not large - just average sized. It was very nice to have access to bath direct from bedroom & not lose any space in either room to a swing door into the bedroom or a barn door on the wall. A pocket door seems easier & quieter to close. In my current home, which was also a new build, we have a pocket door from master into ensuite master bath & also another from master bath to toilet room that is inside the master bath. We love both of them - they are nice, high quality doors, have had no issues with them in the almost 11 years we’ve been in this home.

You locked the hallway door to the bathroom at night? So no one else could use the bathroom? That shows perfectly what is wrong with a jack and jill bathroom.

Locking the hall door to this bath was more a reflex than a necessity in this 2 story home, with master on main with its own ensuite, powder room downstairs, two bedrooms up each with ensuite baths. This guest bath with door to hall had an office across the hall from it & a bonus room down the hall from it - that’s why it had a door access to the hallway. I think the answer for OP lies in what the entire layout of house is & how often they might have overnight guests & who those guests are. In other words, the decision for an access door to hallway & the kind of access set up for the guest room that bathroom serves isn’t one to be made in a vacuum & certainly not a definite No to pocket doors.

Pocket door isn't the greatest option, but I think it better than a barn door. Is there a reason why a more standard door cannot be used?

It’s a new build - are you planning to sell it so soon? A pocket door v. barn door on one bathroom will have zero effect on resale value.

I suspect a pocket door is just harder to install and if you can be convinced a barn door will work, it's a win for the builder.

You can have a solid pocket door installed and it'll be absolutely fine for noise control and whatever (although noise shouldn't be an issue in this setup anyway). I have some, including between my laundry room and TV room.

Pocket door affects switch placement. But it's much better for furniture placement. And it's a better door altogether. And barn doors are trendy--or they were a while ago.

Neither option is good. Skip the J&J in favor of just a hall entry.

Actually for an infrequently used guest suite (or even frequent!), I think this is an ideal layout.

It's a hall bath until, BAM!, it's a guest suite. This provides a great deal of flexibility with few negatives.

A bath shared between two (kid) bedrooms? THAT is not a great plan. But this concept is pretty slick.

I’m on the same team as @Fori for this bathroom question!

Pocket door is much better for privacy and I would think it would be for resale as well. I think the builder just didn't want to mess with it. IMO barn doors are no longer popular, unless you need a door and don't want to tear up the wall to put one in. We have had pocket doors in bathrooms before, and now have one in our laundry room and a closet. I like them. They have their own hiding place when not needed.

What Fori said. (I always agree with Fori, lol.) Pocket doors are more difficult to install. No to the barn door!

We once had a high quality solid pocket door on our master and it was as good as a regular door as far as sound traveling (or not).

I personally don't know if I'd do a pocket door on an en suite again if I had any other choice though because no matter how well it's installed, a pocket door makes a sound when opened and closed that may disturb your sleeping partner. However, as a guest, it would be nice not to have to go down the hall in the middle of the night. Pros and cons - only you can decide.