I would be sorely tempted to put a pseudo-hill/tunnel on one corner (there was an issue of CTT that had a layout with one that was made of plywood, with a flat top so you could put buildings or track on it, and the sides were done with pieces of wood cut and sanded and painted to look like rocks--the result looked like something Buddy L might have made, which would look good with Marx of course). Then I'd add a twist: I'd insulate the track inside the tunnel, and have it activated by an approaching train. That gives you two things: Two-train operation on your outer loop, BUT it also gives the illusion that your trains go somewhere, because they go into the tunnel and they disappear there for a while. Louis Hertz used to do a similar trick with a pair of switches and a passing siding in his tunnels, but with this method you don't have to remember which block is occupied, and with this method you don't have to be at the controls all the time either.
Overall, nice layout. Mine's in pieces right now so I can't share. When I finish rebuilding it, it'll be an 8x8 with an O42 outer loop and some Marx O34 inside. I'll slip a circle or two of O27 somewhere also. I *want* to work in a circle of American Flyer S gauge too, since S and O27 are the same scale (despite the difference in track), but I'm having trouble finding somewhere to shoehorn it in.