Is that the kitchen to the side of the other end of the room? I would definitely make that an eating/game table/homework/projects area, or if you don't need that, make it a separate reading/chatting space for when not watching TV. The two ideas could be combined. Why not put the TV between the doors? A flat screen can go on a console table or media stand and it looks like you could get a good size one there (45-50, maybe 50-60 inches?). Maybe not giant to ginormous, but plenty big unless that space is smaller than it looks. If you don't put a TV there, it has to go on one of the two end walls. That means the TV will either seriously limit your seating options on the stair end (sofa/love seat against the stairs will not be a good viewing angle, so you are probably looking at primary seating needing to float between the stair landing and the sliding doors and leaving traffic paths around it) or you flip and put the TV at the opposite end of the long room and the TV viewing is the cozy nook -- BUT the TV size may have to be limited by the space size. You can't sit on top of a really big screen. If your kitchen is off to the side, consider whether you will want to see the TV from there. It could be great either way -- two great rooms in one. You might have to tame the TV dreams. The guys in my house don't want to live with limitations either. I know what that's like. Best Buy has TV/room size and distance from screen charts that might be helpful and sometimes it helps to mock things up. Don't rush -- taking a little time to get a feel or how you want to live in the space and how it will best work for you can save you from some expensive mistakes.

It definitely looks like you are almost there. I agree with you I think a navy blue sofa will look great with your area rug and the two side chairs you have which already have the same tone of blue as in your rug. To bring the room together after work with the accent pillows in the sofa introducing the red and the golds and blues you have in the rug. And keep a sectional!

If you have a sectional in mind, I might go for more of an L shape instead of a type with a chaise. And instead of covering your built-ins, I would place it so the shorter part of the L back is along the dining area to create a division there, and the longer part of the L is along the wall opposite the TV. This way you wouldn’t be blocking the entry doorway to the living room. You may even be able to keep one of your chairs with a small table (in front of your window).

Nice room. Liking the large window. Might consider adding plants on a table or plant stand behind the seating if the window gets any direct sun light. If the seating shown (which looks really good) is comfortable, keep it in lieu of a sectional. No one really likes sitting for an extended time without arm rests. Sectionals limit furniture placement options. If any one part of a sectional gets damaged or stained, then you either live with it or must replace a lot of seating. Choosing pieces and/or placement to get the seat closest to the fireplace a little farther away from it might be preferable -- at least if the fireplace is actually used for heat (likely better for the life of the furniture piece). Would likely want the longer/wider one (with more seating) facing the tv/fireplace UNLESS you expect someone to regularly lie down on it to watch tv requiring anyone else to watch from the other seating at a more awkward angle. If you are considering yet other options and budget is not an issue and you don't usually seat a lot of people, you might consider investing in several very comfortable size appropriate recliners (rather than a full sofa or a love seat or a sectional) and place two of them side by side with a narrow rectangle table between them, then add a round or square table on the other side of each of the recliners so you could add a pair of recliners across from one another ... each at a right angle to the two side by side recliners.