My living room layout is very similar! I had my couch horizontal in front of your window creating a entryway, maybe put a console table behind the couch.

Do you have plans for the niches next to the fireplace? Maybe builtin storage cabinets and TV on a pull-out swivel on the right. The L side you could leave blank or have shelves above the lower cabinet. It will add some architectural interest and allow for storage. Then a seating area in front of fireplace - sofa and 1-2 chairs. A small desk area in the space between sofa area and dining. Though I’d probably have a computer station that closes off (do you need for desk top or just lap tops?)

You do not give any measurements, and that makes it impossible to envision. Measure each wall in the room. Measure from a corner to door and window frames. Measure the width of the door and window openings. Draw out the measurements on paper so that one foot equals one inch. You don't have to use graph paper, but use a ruler to keep the scale accurate. Position the door and window openings as accurately as possible, and if a door opens into the room use a curved arrow to show which way it swings. When everything is drawn out look for obvious pathways. Since you describe this as spacious, there should be room to walk around furniture groupings. Next measure your furniture—sofa, chairs, tables, bookcases, TV, etc.—that you plan to use in this room. On a separate piece of paper draw the outlines of these pieces at the same one foot equals one inch scale. Label each furniture piece and cut it out. Now you can lay these furniture pieces on top of the room floor plan and move them around till you like what you see. Here is a picture of one room plan. As to where to put your TV, many people put it next to the fireplace so as to keep the focal point on that wall. If there is not enough room there choose another wall. Don't place the TV in front of a window, especially if you watch TV during the day, because the backlighting causes eye strain. All the guides I have recently read say the bottom 1/3 of the screen should be at eye level of a seated person. These guides also recommend sitting much closer than I would have thought, and that distance varies with the resolution of your set. For instance, a 40" 1080P HDTV is best viewed between 5' and 8.3' away, whereas a 40" 4K Ultra HD TV has optimum viewing between 3.3' and 5' away. Here is the link to that info. That seems ridiculously close to me. Should you have no suitable wall for the TV consider using furniture as a room divider to hold the TV as below.

Hi. I've reproduced your room in a 3D build to help you visualize in a better way. To maintain a foot flow I have centered the sofa facing each other. You can add one or more swivel chairs which you can tuck away or corner them when not in use. Mounted tv on a wooden panel opposite the fireplace. This way it serves as a family room as well as an entertainment room. Perfect for a winter evening when it's too chilly to leave the fireplace but also wouldn't miss your favorite show. If you can get rid of the piano, you can place two chairs and a bookshelf and create a little reading space. Let me know if you would want me to work on more layouts. Always happy to help!