What a nice wall area you have to work from. I like the framing that goes up the wall from the fireplace sides all the way to the ceiling. I think the reclaimed wood/beams idea would be very good there as it would be more in scale with the size of the room.

I would use those vertical framing pieces to help draw the eye up and have that be a focal wall. Paint that framing in a color that would accent your color scheme, add your beam mantle and wood for the surround. Shelves should go to the sides of the fireplace paying attention to their width on the window side of the wall. I might tend to limit their height to the top of the door frame. Adding any ceiling beams?

everything in this room is too dark. I would not add more wood, as it would also be dark and heavy, thus adding to an already gloomy room. Slip cover or replace the furniture in lighter colors, get the tall mirror suggested earlier and paint walls a lighter color, avoiding the browns. Colors and lighting are needed to dispel the gloom. rustic wood on the wall will only compound the problems. I admit I do not understand your desire for a "rustic modern" look. I gently suggest that rustic and modern are a contradiction and do not make the room more livable or attractive.