IMO wing chairs are timeless and you can use almost any style of sofa with them -- just make sure the size is compatable (no overwhelming HUGE sofa or teeny tiny dainty one either). It doesn't need to be leather. In fact, I think it would look better if it wasn't, but that's me -- I don't like all the furniture in the room to be the same.

I would go with a cloth couch and add more texture. An area rug would break up the browns as well.

I would go with fabric (no tufting) & any colour could work since you have such a blank slate. Usually I try to keep the seat height of the upholstered pieces about the same,since your chairs are wings they will likely be taller than the couch. Have you considered picking your rug first and then building up from there? For me,it is easier that way. I find the rug sets the tone and colourway.

Hancock and Moore has customizable loveseats. I think you need something that will give the same visual weight as the chairs, but not tufted, and in a textured fabric. Repeating the arm style, or at least the nailhead trim, will tie the sofa style with the chairs without being too matchy-matchy.

http://www.hancockandmoore.com/Product-Category/16/Your-way

An example, but not this color or fabric

I will differ and say I would get leather.....it is so easy to care for and comfy in all kinds of weather (here in the snow/hot midwest). I like the picture suero had for a couch and I think you could use the dark brown like the chairs or even a beige color. A rug will look nice when you get the lamps tables and everything in your room.

Oh and make sure the couch has a sufficient height that is similar to the chairs