It looks like you have some minimal trim in the front room. You would not go wrong matching that. I wonder if the larger opening was a modification because it has different casing on it?

Is the paneling the brown on the far left? Is the current trim the same color as the walls? It looks like there is limited trim. Perhaps stain the trim the same color as the beams to add depth?

@palimpsest thank you for interpreting! I am but a lowly homeowner and don't know all the correct terms. :) Yes, that is all correct - thank you for the graphic. Yes, the contractor believes the larger opening was once enclosed as you suggested. After staring at that little bit of wall for 13 years, I'm ready to get rid of it. I don't need to match the existing trim because it is all in poor condition and will eventually be replaced. New wood floor is also on the agenda since it is only a 1960s linoleum underneath. I'm probably going with a wide-plank white oak to not compete with the dark beams. Do you think I should finish the jamb of the new opening with millwork or leave it a drywall jamb?

@Cindy Bergstrom no the paneling is on the wall in the forefront painted grey. Here is another angle from the other side. This 26" wall in the center will be removed and replaced with a drywall opening. Do you think I should finish the jamb of the new opening with millwork or leave it a drywall jamb?

If you know what kind of trim you will be replacing it all with, and you are going to get started on that, I would trim it out. If you don't know yet I would make the opening drywall.

I would definitely finish it out with new trim. Decide what you are going to use for trim, when you replace the damaged trim, you have. I would try to match the beams with the new stained trim, all through the house.