Went back to the drawing board recently, after attempting to design a mid-sized island layout with wider curves, and decided that many of you are right and that an around the walls approach has its advantageous. 
With any luck, I may actually be able to build this over the next year or two. I have access to a roughly 15x15 space (the space is a little larger, so I could fit a 15x15 maximum layout footprint) in the basement, with walls on two sides, and open access on the other two. A large area in the middle would be left open. I should be able to reach everything. Maybe liftout bridges or a hinge for access from the left side. I am able to duck under, so that is an option, too.
The outer main is O84, and the inner main O72. The yard is O60. I think there are options here. Could go even wider on the curves, or keep both mains at O72 to save a little space and try to reconfigure the yard so that:
1) The arrival/departure track/yard lead/drill track combo is O72, allowing any train to pull in and arrive/depart. I suppose this track also serves as a passing siding off the inner main.
2) Provide an O72 pathway to a track or two where O72 engines can be parked. I don't have any, yet, but that may change.
3) Depending on the main curvature and benchwork, there is room to insert some industry sidings around the layout, too.
I intend to mostly operate freight trains, with diesel power. I am sure O72 steam will make it into the collection eventually, but I see most of my purchases and operating going diesel. Other than just running/watching trains, I think it could be fun to simulate some operations by having a road freight arrive/depart from the yard, while a switcher can move cars around, build a train, or run a local to serve industries.
I designed the yard from Ken's trackplan book and some other forum resources. I think it would be functional for assembling or disassembling a train without impacting the mains, allowing switching to happen as an activity while trains run. These would be freight operations - not really enough room for passenger trains. Maybe it is too ambitious, but I like the idea of trying to work in a yard and I think I have just enough space for it. Enough space for some engine storage as well, which will be mostly diesel power.
The outer main could be slightly elevated, reaching 2-3" in the top right, with a ~2% grade on either side. It would not be a huge scenery layout, but there is room to tuck a mountain or tunnels somewhere, and perhaps a narrow town/city scene at the top of the layout.
I'm no carpenter, but I would be able to handle the benchwork for this. It would be open grid with plywood, and maybe homasote. I would likely try to assemble in modules, so that I could dissassemble and reuse if say, in 5 years, we gut and finish the basement...
I put this together in FasTrack, but it could be redesigned, and probably better, with Atlas or Ross/GarGraves. Cost to build, using FasTrack, would be roughly $2,500 based on buying everything new, at sale/discount prices from dealers. Could probably shave some of that off by purchasing some of the track used. Probably another $1,000 for lumber, wire, and related layout building supplies. Countless hours of time that I don't have to build it. 
Anyway, appreciate the feedback as always. I think I am getting closer to a style/idea for the next build.
Two screenshots attached...these are largely the same, but the yard lead/drill track/etc is a bit different. Trying to find the most efficient/effective setup for that.
