You have a nice space and a nice concept. I always like a nice viaduct running over a railyard and industry. The issue you deal with in you limited space is the mainline is always on an incline, making it hard to add turnouts and sidings.
Here are some other ideas:
The first plan creates a second loop and makes the layout a twice around. The inside loop is flat, solving the incline problem. I also show where you could put connectors for a reverse loop. It does require a little more space on the front left side.
Plan 2 just pushes the mainline into the corners, to open the space inside the loops but you have to deal with the incline problem. Enjoy...
