You need to add a 6" wall on the left to be able to create a cased opening.

Wall in red.

New trim in blue.

A cased opening like chispa shows is your best bet. You would probably need 2 2x6's and a 1/2" sheet of plywood or a double layer of drywall to be the same size as the 2x4 wall on the other side.

What style home is it? Crown molding and wainscoting are very traditioanal. How much trim is around the windows? I think we need a few more pictures. Also IMO you would have to case all the doorways on that floor of the house.

@jck910

The home is a 1990s colonial. it is traditional in structure, but seriously lacking in character. We are tackling the whole thing! We just refinished the floors and put wood on the stairs. I like a traditional feel and would like one of the rooms off the foyer to be a library and the other to be a formal dining area. Here are other photos…all from previous owner.

The previous owners really got caught up in the "gray" trend!

Are you willing to follow this new trim to the whole house or at least the main floor. IMO trim added willy nilly looks eaxactly like that was done. First does your decor suit all that trim ' you say you like traditional but does your decor say traditional. let's see your stuff and what you have done so far. Pist pics here just like you did already .

It doesn't have to be trimmed either. It can just be done with drywall to match the other side and then add matching baseboard to it.

Adding some more trim in a house that already has, what looks like, decent trim isn't "willy nilly" ! Casing that opening and adding crown in a few rooms isn't going to dramatically change the style of the home, just make it look more finished.

Wainscoting might be hard to tie into the staircase in the foyer, but would certainly work in a living or dining room.

When looking at crown, also look at its simpler version, cove molding.

It may end up being just drywall in the foyer to do the separation, but I will be definitely be adding some wainscoting…simple, crown molding, and perhaps wallpaper to the more formal dining room. The future library will get crown molding too. It has a large great room that will also get some form of molding over the fireplace…maybe shiplap, as it has vaulted ceilings. All the trim is being painted pure white. My kitchen cabinets are going to be pure white with a wide beadboard backsplash and a granite that looks like soapstone counter. Pretty simple, but I’d like it to be cohesive. I don’t have a lot of photos because we haven’t moved in. All floors are being refinished.This is what I’ve got…shoji white as primary home color. Making progress…..

Also, realizing stair rails look orangey next to new stairs and refinished floors. Do I paint them? I was thinking maybe sw black fox, as I wouldn't want it too stark.

@chispa Thanks for the tip on cove molding….I like that it is simpler! May fit this space better. I don’t want ornate.

Def not a pro but i feel like black would call attention to the railings and I wouldn’t want to do that, so I’d just paint them white so they disappeared, but i am admittedly a low contrast girl :)

@HU-285050505, this is my cove molding, just a simple curve with flat ends. I used the 7.25" size, because I have 10 to 12 ft ceilings. They also offer a 5.5" size.

IMO a poor way to handle the opening at construction time. So yes to building out that left side to create a proper looking opening but the space does not look to be one where wainscoting and crown moldings belong. IMO that gets added everywhere or no where it looks tacky in just one or 2 spaces.