One of our customers had a yard a little similar to that. It was a lot less sq. footage. But it was similar in the sense that before we came in, it had a slope toward the driveway and toward the sidewalk. Nothing but grass. And the grass was unusable and not attractive. So we flattened out his yard with a nice segmental retaining wall. Added some outdoor lighting, added a nice level lawn, some ornamental planting, and a small rock bubbler fountain. This is what it turned out like (below). You could do something similar, just on a little larger scale.