I always like to start with a few inspiration photos to help give some visuals of my ideas for your space:

I love this gallery wall layout with the assorted prints + taxidermy around the cabinet.  I was thinking something like this would be perfect on the wall around the antique cabinet you have there.  We could also add some objects/accessories on top of the cabinet to create some height up there.

I love the overall feel and style of this living room and the simple styling above the mantle.  I'd love to add some kind of curtains or maybe a single draped panel on your windows to soften the space.

I really like the style/colors of this room too and love the large trunk coffee table.

I know you mentioned possibly switching out your existing armchair and I think you have plenty of room for either a pair of matching chairs or a larger upholster chair with an ottoman if you wanted. I also like the simple styling of the neutral/rustic space above.

I'd love to incorporate a few more furniture pieces into the room- some side tables and a coffee table in particular.  Some tables with lots of character like these reclaimed wood pieces would be perfect for tying the entire space together.  I love the overall neutral but cozy color scheme and style of the space above and how they layered the photos on the mantle.

This photo instantly made me think of you long console table which we could style like this with a few accessories and a lamp or two.

Simple but cozy styling- love the coffee table and the matching chairs.

I love the rustic but cozy feel of this room and the cowhide rugs.  I thought with the overall style of you room, especially with the taxidermy and equestrian art, it might be neat to add a cowhide rug to the sitting area or layer on top of another rug too (see next few photos for rug layering examples).  

Love this unique and eclectic gallery wall space and the pops of blue in here.  

I love the simplicity of this room and all the texture and warmth.

A little more formal than I would do for your space, but I love the colors and equestrian style.

Loved this mini equestrian art gallery too.

Another equestrian gallery wall example.

I love this colorful printed rug in here and the cool industrial coffee table.

This is a more neutral space but with lots of different textures incorporated for lots of character and a cozy feel.

Another equestrian/ taxidermy gallery wall mix and I really love this reclaimed wood coffee table too.

I love the brightness of this space and the pops of blue- would be such a pretty color scheme for you room.

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Adding some larger framed pieces to your walls will add dimension and fill the space in nicely- I love the look of the antique gold frames.  Below are just a few shopping suggestions to get us started- let me know your feedback and I can continue to add more options as needed.

Below are my initial thoughts on the arrangement of the room.  I think if we set up the television on your long console table and centered it on the right hand wall it would really help anchor the room and define the space (we can also use another piece of furniture besides the long table if you prefer, but I do think it needs to be some kind of console table or cabinet large in size).  Then the leather couch could move to the left side directly across from the television.  This would give us room for one or two chairs facing the fireplace and angled towards the center of the room.  Having chairs facing the fireplace instead of a couch will also open up the room and give you better sight lines of the fireplace.  I would center the cabinet you have on the left hand wall and create a gallery wall around it and above it.  We could definitely use a slightly larger piece of furniture other than that cabinet if you wanted too, as you have plenty of space for it and that may seem a little too small on the open wall, but we can make it work either way.  

Below are just a few shopping suggestions to get us started- let me know your feedback and I can continue to add more options as needed.