Here are some photos. We are trying to decide if we should keep the sectional (we do love it, but it needs to be cleaned or possibly re-upholstered) or get something new that will fit this space better.

Oh, looking at your pictures, realize I misread your diagram. The room is smaller than I thought, so my first idea isn't workable. Still a beautiful space though! Reupholstering is pretty expensive. I would buy new smaller. It will cost the same or less as reupholstering a piece that large.

Rethinking ideas. What is the distance from the fireplace to the opposite wall? The straight wall, before the turret starts. And what is the length of the short outside wall that runs from the fireplace to the turret?

I don't think there's much real space in that corner that you can reclaim. I think I'd put a small table back there that could hold a table lamp, the plant, and maybe act to hold a few things for anybody sitting in the middle of the couch (magazine, drink)... Maybe you could put the round table there, to unblock the bookshelf a bit, and get a narrower end table for that end of the couch.

Nice looking room!

Agree with acm's suggestion as best way to use existing furniture. Long term, that's still a good arrangement but a smaller sectional would open up the room more, let it "breathe." Or a faux sectional--matching arm chair and loveseat positioned in a tight L with arms touching, end table filling the empty square behind the intersection arms. I'd finish with a small-scale chair near the fireplace on the entrance side of the room. And a larger plant behind seating in the turret--a tall potted plant or a lush hanging plant.

I just found these photos online of an old real estate listing for my house - I'm kind of digging this diagonal furniture arrangement and thinking it might work for us!

Agree, that looks really good! A much lighter and open look.