I can't tell from your picture if the side of that piece has been finished -- meaning polished a bit. It should match the degree of polish that is on the broader face of the piece, not rough or raw cut. If the side has been polished to match the face of the piece, I'd just leave it. The corner edge could be rounded slightly, but beyond that, putting more around it would just be drawing attention to an area that should fade. Isn't the rest of the shower what you want to show off?

You could put a marble pencil molding all around the doorway. It would protrude beyond the edge of the marble jamb, providing a little bit of interest with the relief and shadow. What is that red stuff?

That is Red Guard. I think it's a waterproofing paint. We have it in all our showers. Is it not the right material?

I believe the edge of the threshold pieces are all honed evenly. To me, as it is, it doesn't look "finished." The pencil liner is a great option. Thanks.

RedGard is fine. It's just a very sloppy job.

Yes, it is sloppy. Might paint cover it if we don't end up adding stone?

It looks thick and gloppy. You'll have to somehow smooth that out. Although it's too late now, I would have preferred to see the waterproofing wrap around the corner. That area just outside the curb is very prone to water damage.

Can you post one of the Houzz pictures so we can see what type of look you are wanting?

Even if you have the drywall guys pad out the wall, how would you handle the baseboard? And the marble frame would stick out further than the marble on the curb. If you use a pencil liner, don't forget that you will see the thin set where the pencil liner meets the around the frame.

Maybe it will look better when the drywall is evened out and the Red Guard is painted over? Anyway, here are some images from houzz:

[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Oakville Photographer Arnal Photography

[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by San Francisco Architect Taylor Lombardo Architects

I never believed this when I was in the midst of my reno - but little things like this bother you SO much now, while later when it's all done, they don't. I would leave it as is.

The first picture has a wood frame that goes all the way to the floor. The marble curb is inside this frame. The second picture looks like yours except in yours the marble on the sides stops at the curb and in the picture the curb is inside the side marble. It would be a lot of work to tear yours out and get it to look like either of the pictures.

You could use marble casing, Just have them rabbett the edge of the marble casing as needed so the rabbett will envelope the marble that is protruding past the face of the drywall.

The difficult part will be integrating the added casing with your marble curb and baseboard.

Th easiest would be to use a narrow strip of marble casing (I'm guessing around 1-1/4" wide from the photo?) that will simply sit on top of the marble curb and not interfere with the baseboard, but that might be a bit delicate. It'd be right along the same idea that was previously given of adding a pencil liner.

The slightly more difficult, but more appropriate, would be to put in a wider casing in the 4" to 6" wide range and cut back the baseboard to accommodate the wider casing.