Ok, in my opinion, I would not put the tv on the the wall to the left due to the window reflecting onto it, unless you want to use blackout window coverings which you would have to keep them closed most of the time:/ Instead, I would replace and lower that mantle with an appropriate sized beam for an updated look and then you would also be able to to lower your tv a bit, then add some floating shelves on either side of the fireplace that would also tie into the beam mantle. I would need to see more pictures of the whole room to help with more ideas. Hope this was helpful even though I’m a couple years late on this post, 🤪

Think about furniture placement. You may not want to put the fireplace centered in the middle of the wall, as it appears your furniture won't be able to be centered in front of it. That bump with the 4' opening is for a door, I assume? Where does the door lead? You will need to leave that whole area along the stairs/living room wall open for a walkway (leaving 42-48" open along the wall would be ideal, IMO) to walk through the living room to the door. If you center the fireplace on the wall, the furniture facing it will need to be off center to account for the walkway. The window and door bump outs are going to be awkward to furnish/decorate around, I'm afraid. It looks like the fireplace wall is just under 17', but after accounting for leaving open space for the walkway, that gives you just under 13' wide of space to work with for arranging furniture in the 10' area (the length of wall with the 3 windows) in front of the fireplace. Or are you planning to put your couch/es in the area where the room widens (the second trio of bump out windows)? If so, how are you going to use the ~13x10' area in front of the fireplace?

Is the living room fireplace a wood burning fireplace that you will actually use? Is the cover in front of it a solid fit that will keep any warm/cool air from heat/AC system from going up the chimney? Since the living room is open to much of the rest of the house, consider having the living room fireplace converted to a gas log fireplace. No "smoking up" the more formal living space. Put your more formal living room seating area in the front half of your living room. You could put a (round) table with chairs (that could be part of indoor/outdoor living and entertaining including the patio or, since it adjoins the kitchen, even be used as a family breakfast table) on the patio end of the living room between the fireplace and the door to the patio. Rather than create built ins in the formal living room, you might consider enlarging your door to your patio and centering it. Alternately, on the exterior wall nearest the kitchen, you could create a home office area for your computer for communicating and meal planning -- even to be able to hear the news while you're prepping a meal and, instead of built ins, add furnishings suitable to how you will use that space -- perhaps including a barrister bookcase. Is the family room fireplace is a wood burning fireplace that you will actually use? Is there a cover in front of it that is a solid fit that will keep any warm/cool air from the heat/AC system from going up the chimney? Since the family room can easily be closed off from the rest of the house, keep that fireplace as wood burning fireplace but do add a suitable cover to the firebox to protect against having a spark start a fire. In the family room, you could paint the beams the same color as the ceiling, however, unless it is damaged or there is another issue, consider keeping the paneling. Would put the tv in the family room on the wall between and below the two high windows opposite the fireplace. If the room is too dark, you could add light painted) colored thin wood (luan) panels to the wall behind the shelves without painting the walls ... or removable peel and stick wallpaper -- without damaging the value of the wood paneling and, potentially, damaging the resale value of your home. The base cabinets of the built ins could be perfect toy cabinets. You could also add doors to cover the shelving above and create yet more hidden storage. Is that brick flooring actually vinyl rather than brick? Unless you truly hate the current family room flooring, consider keeping it at least until your children are older so that the family room can be where the children play without worrying about damaging the flooring. Adding a camera to the family room could enable you to watch over young children from anywhere else in the house. Using the family room as a play area means you could focus on comfortable furniture there and put your more upscale furnishings in the living room and dining room where guests will more likely be entertained.

We had replaced all but that "room" of windows with white vinyl. My husband and I couldn't agree at the time, with what to do with them. He wanted to use wood look vinyl. White vinyl is sort of our compromise as I feel like it's better to not match at all, than have it be close, but not quite. Is far as why not add a door, I guess because we usually have furniture in front of the windows and there is a door just to the right of those windows, in the kitchen, so another exit seems redundant. The room is such that furniture really needs to be by those windows, or there isn't a place for it, otherwise.