If that is a 4X8 footprint, you don't have the length or rise-over-run for a grade to get to a nether level. It's not that you want to have the two levels and lengthen your main line, which this will have to afford you to close the loop, but you must be able to actually operate that lower level, to see it, and to fix problems under there. We'd need maybe a solid foot of elevation difference, and with that small separation, you'd hate being on your knees all the time to figure out what you need to do to enjoy yourself. Trust me, the physics in the scale world works out exactly the same, with the same problems and challenges, as in the real world.
For the track plan you're showing us, what you see is all you should get. You can add a bridge on a curve someplace, a small hill, maybe build a partial mountain backed into one of the corners with two portals...that kind of thing.