You've got a lot of space but not a lot of furniture to go in it. Build yourself a monster-sized couch for a fraction of what you'd spend on a large section using this clever DIY method.
Core77, a design magazine and resource, highlights a problem one of their readers had and his DIY solution. He rented his photography studio to a client without asking enough questions and a particularly vigorous porno-shooting session caused irreparable damage to his lobby couches. In a bid to build bigger, better and cheaper couches you built a set of banquettes. Banquettes are essentially upholstered benches and often arranged like sectional couches.
Using little more than plywood, upholstery foam, and some basic carpentry skills—he outsourced the sewing—he created huge and comfortable couches for his studio. Check out the full guide at the link below for copious photos and sketches—he was serious about getting the design right—as well as tips and tricks on what not to do.