Been busy at work/home, but I appreciate the comments and continued feedback. Took a few days away from AnyRail, but earlier this evening while lying on the floor next to my toddlers bed while he refused to fall asleep, I had an idea. This was spurred from @gunrunnerjohn layout - his multi-level folded dog bone where the ends fold under themselves. But, I've adapted that concept to the L-corner within that 15x17 envelope. It's not perfect and needs a little tweaking to make everything fit properly, but fundamentally it would work (I think). It's one continuous loop, around 130 feet run, and all O72 minimum. There is room for larger curves in some places and that hasn't been tweaked/optimized much yet. Figure with 130 feet, you could at least run 2 trains, maybe 3. I suppose there are some limiting factors of having this as one continuous loop, such as train safety.
If you started on the outer in the bottom right corner, you would begin upgrade and eventually reach the top, crossing over the lower level in the top left corner, then descending back down along the right side of the layout. The lower level then continues around the dog bone, passing under the elevated section in the top left, and then returning to the bottom right of the layout, for the next run upgrade. I think the crossovers act as reversing loops, so trains can be turned around and run either direction on the layout. I elevated to 6", but perhaps I should go higher? Would need some advice on how high to go and what the clearance should be for bridge/tunnel portals. The grades I casually worked in just now are around 2.5%. Not hugely important now for this planning stage but looks feasible and that's the main thing...
I'm not an expert carpenter, so I've shyed away from multi-level and grades, but with a space this size, and that my main interest is running trains, that the trains need somewhere interesting to run. I'm sure I could figure out the lumber...or get some professional help. A plan like this might be a real contender for this future layout. I think the visual setup is "right" here as well - the grades and elevated sections would be toward the back, while the lower level is toward the front, so the viewing should be good and there are several opportunities for tunnels.
*Yes there are some access issues as pictured but I think I can work with the space and layout to mostly avoid those in reality.
**No yard at the moment but can probably fit one somewhere on a lower level or off to the side.