How about a desk in front of the built ins, which is nice because one can easily swivel the desk chair to get things off the built ins. You could also house the filing cabinet inside one of the drawers. This may require some creativity in selecting said files to fit in the space. Placing the seating near the fireplace but facing the built ins and house the tv in the built ins as well. If the tv is placed high enough you can easily see it from the seating area.

How about placing the desk perpindicular to the window in the built-in so your back is to the wall when you're using it? If the recess right of the window has nothing in the bottom, try a file cabinet there. Angle hubby's chair for watching tv/gaming facing the tv opposite the desk, so you can see each other and converse. Sofa and two armchairs in front of the FP at the other end with a coffee table. I think you need to turn this rug and bring it towards the FP for that furniture to sit on. Find a second rug for the office end - doesn't have to match, but try for similar style and colors, or use this one elsewhere and buy two new ones, maybe same colors but vary the pattern or to match - your choice. homedecorators.com would be a good place to shop for one.

I think you need to have the built-in reworked to house filing cabinets and then place the desks in that area. That frees up the fireplace area for relaxation, with TV on wall that is to left of the door by the fireplace. Place sofa facing fireplace with 2 chairs to right of sofa, flanking the fireplace and facing the TV. Coffee table in front of sofa and a round table between the two chairs.

If anyone has read a number of my posts on living room furnishings, you may have noticed (tho probably not) that I usually put a round table between two chairs. Two reasons:
1. It's easier to angle the chairs with a round table than with a rectangular or square one.
2. With all the rectangular items that are in a living room, I like to break that up with a round piece or two or three.

However, when I place two chairs close together and set straight, not angled, then I would select a skinny rectangular table to fit between them.

I'm not a big believer in end tables, preferring instead to use floor lamps instead of space grabbing end tables. Of course, in huge rooms that's not a problem. I always try to give everyone seated in the room a nearby place to set a cup of coffee or other beverage.

I like the way you have divided the space into TV watching and office zone. Perhaps have the desks facing each other would give you space to walk around while moving the TV to the right of the fireplace wall would give a better viewing space.