Holy smokes, Paul, your space is very similar to the shape of rooms I am building my basement layout in! It is not a large basement but I have to say my plan utilizes the space very well. I have a wall and steps separating the large room from the smaller - and I "tunneled" under the steps in the interior part of the basement and used bridges and cantilevered shelves along the outer walls (see diagram). I have a peninsula connected to the outer mainlines via a double-crossing (4-45 degree crossovers). I went with 43" base-height - allows for comfortable viewing and working under layout. My friend, whom is a carpenter, helped me build the curved sections - this allows for smooth walking between the aisles and is pleasing to the eye IMO (its a lot of work though).
I designed this on engineering graph paper and did all the calculations for track diameters on scratch paper. I attached a floor plan and have yet to pencil in the tracks! Long story short: I used maximum diameters on my mainlines (O-72 min up to O-128 max - Ross track) and had to use a lift-out bridge that separates the smaller room from the utility room and workshop. I am not suggesting you use this approach, but it is one fun layout because from every viewing point the trains appear to disappear on a journey and it is the only layout I've ever built where watching trains doesn't become boring after awhile. It took me quite some time to design it and there are some unconventional aspects. Since these photos were taken I've built a large city on the peninsula and am currently working on the outer "shelving" areas. The smaller room will be a "Christmas in the City/North Pole" fantasy" area.
Here's some "under construction" photos of the tunnel, peninsula and cantilevered shelving:
