If this were to be my house the first thing I would do is paint the fireplace the color of the wall. It looks rather dated and also makes the room look smaller. If you were to put the entertainment center in the corner it will compete with the fireplace and chop up the area even more if the fireplace stays as it is. One of your first priorities should probably be to decide on a color scheme so that you'll know what to look for in terms of drapes and a carpet. If you find a carpet you love you can pull out different colors from it for your furniture and drapes, which might simplify the process. Having a lot of dark furniture with dark floors might be too much unless you're only talking about wood furniture and not couches and chairs. Just remember you'll spend a lot of time in this room so take your time to pick items you really love in colors that make you feel happy.

What a great home you have to start with!! I love the look of modern furnishings in traditional homes, below are some houzz photos that came up with that search. IMO it adds instant warmth and uniqueness. I think sofa and accent chairs will give you more flexability and a more open sight line will be achieved with a low back sofa. I think the art and decor you should take your time with and let it speak to you and your families interests and tastes...is the art modern and artsy, or classic with a love of nature, is it fun and colorful with a nod to the bold. The backs of the built in bookcases could be wallpapered to reflect this too, and lighting will add a huge statement about you and how you want others to feel in your home.

When was the house itself built? The interior design aspects are on point with the mid 1980s but the architecture is earlier mid century. I can't really picture someone building a house in that style and decorating aspects of it in late Modernism/post Modernism at the same time. I can picture someone updating a 1960-odd house this way.

If you lucky right now you might be able to match the flooring if you wait you will not Even if you don’t install it right now see if it is still available and get enought to do it later , they get discontinued often.As for the rest do not let the gray walls dictate your choices , area rugs are IMO much better than wall to wall always. I really suggest you do not try to figure out your style get what you love with the help of an interior designer if you can find one that can work with you to slowly do your space. I have had clients that we took over 2 yrs to finish a space that was theirs totally. Since right now is not a good time to shop work on your idea books it will give us a better feel for what you like and if you hire a designer it will help them too.