You can add a lot of color and brightness through the use of accessories. You can add them all in a single color...as here they focused on the coral... ...or gold ...or any other color, preferably a bright one. Or you can add multiple colors such as aqua, orange and acid green as suggested by this design seed. You can even choose stark white as your accent color. Regardless of color, take a look at the pictures above and notice how much interest is added through texture and pattern. And notice how the window treatments are used to reinforce the color theme. Especially in the first one, notice how important the color in the art is to the room as a whole. Here's a room that is all white, beige and brown, yet notice all the textures that keep the room interesting...pattern on the rug, another on the throw pillows, texture on the white pillows, wood, ceramics, pattern on the window treatments, leather on one chair, velvet on the other texture in the sea shells. So there's a lot you can do with brown to make it bright and keep it interesting...just choose carefully what else you put in the room and the brown will become a neutral background for whatever color story you wish to tell.

Hmm... I was going to suggest a more substantial sofa, but now that I see it would block the fireplace if you did that. I also think your couch adds a lovely bit of color. But I love the new couch you're considering, actually almost put it in my home. So it certainly wouldn't be a bad choice. Every good MCM living room begs for some leather, but I think youd be better served adding it as a bench under the window, or in the chaise. I would keep the same chair for the second, since you will already have a mismatch between couch, chairs, and corner chaise. That's obviously a beautiful console but you'll need to measure carefully. It looks pretty long at 80"- how long is your wall? You also don't want anything more than a few inches taller than your chaise. I would pick the chaise first and the console second so you can get the dimensions right. Here are some options for a bench for under the window, and a leather chair/chaise for the corner.

My living room is 17 1/2 x 15 1/2 I don't find it too large for a single seating area. I guess it depends on how you lay out the furniture. Where your TV is we have a fireplace with book shelves. The TV is in the book shelves with 2 chairs flanking the fireplace. I have my couch where you have your love seat and my love seat where you have you couch. Maybe switch your love seat and couch. This would push back the room and fill up the empty space.

Wall art and a staged coffee table are the only things you need. I like your uncluttered space. I like your rug, your drapes, floor plant, couch, pillows, wall shelves..... Browns, cream and relaxing blues- How much vibrancy do you want in this space? Bold or serene? Wall art is personal and very difficult to suggest for another. You could DIY an abstract or enlarge 2 related photos and hang side by side over the couch. Frame them with blue.mat board. A larger painting like this one is an easy DIY- acrylic, which is easy to paint over should you want to add different colors to your room. Give your art a frame by using wood trim ,to the sides of the canvas :https://www.homedepot.com/p/Alexandria-Moulding-WM-267-1-4-in-x-1-3-8-in-x-96-in-Pine-Lattice-Moulding-0W267-20096C/205917028?mtc=Shopping-B-F_D30-G-D30-30_20_MOULDINGS-Generic-NA-Feed-PLA-NA-NA-&cm_mmc=Shopping-B-F_D30-G-D30-30_20_MOULDINGS-Generic-NA-Feed-PLA-NA-NA--71700000032209454-58700003833955726-92700049583831712&gclid=EAIaIQobChMIisfy5PzH6AIVmK_sCh3UPwoOEAQYDCABEgInOfD_BwE&gclsrc=aw.ds You could go bold color, but high contrast color may make your space feel smaller Here is a series of same motif pics in same color palette... Display two rows of pics over your couch. They will invite conversation by guests as they reflect you