My first idea is to use the space to do something really panoramic using two of the walls and one of the "lobes" of the room. The other "lobe" would be devoted to all the support facilities, storage, reversing, workbench, kitchenette (what a lovely word...), boxes full of stuff, etc.
Like James I too suggest bridging the stairwell with simple double track (boxed in, close to the wall) so that trains can simply run around the room without reversing and/or get back to storage without having to appear on scene again.
I also suggest not fixing the main scenic area to the walls but having it freestanding with a usable access space behind, maybe 768mm wide. This would do a lot of good things:
Access from both sides allows the scenic baseboards to be wider than most people can manage. (You have the room so why not use it?)
Moves the layout clear of the mansard slopes.
Backscene can hang from ceiling (cloth or vinyl) some distance behind the scenic baseboards allowing clever lighting and thus avoiding the problem of shadows of scenic elements being cast on the background.
Vastly increases the photographic potential.
(I like some of the suggestions above about modular design and movable reversing loops as the layout expands, BTW.)