I have carried some heavy loads with my Y-frame on 20" Big Apple tyres.
Suspension would make it easier to ride over large bumps and holes but is not worth the added complication and weight in a trailer that can pack flat.
A suspended platform would have to level out under load, so be tilted when unloaded.
I'm not sure what you mean by leaves in the frame, although I do have a car trailer that uses a half a leaf spring rather than a whole one so that would be an interesting design, only supporting half the leaves. And, of course one can have torsion springs and coil springs.
I'm actually planning on doing some extensive mods to this little HF trailer. Hopefully cutting over 100 lbs off of its weight.
On my current cargo setup, my wheels are supported on both sides by the frame, and I like that setup both giving protection to the wheels, and a solid bolt surface. That means a spring would require essentially a double frame, one frame for the wheels, and one frame for the bed. Possible, but very redundant.
I could use a "through-axle-hub" and a solid axle which should be plenty strong if large enough, leaving the hub unsupported on the outside, but easy to attach to traditional springs. However, all the steel stuff including springs on that trailer is pretty heavy.
Anyway, at this point I'm leaning towards going unsprung. I imagine that if I put say 300 lbs on my trailer, then a pothole would put the trailer and wheels under quite a bit of stress. However, the heavier I get, the slower I go, so I won't be hitting any potholes at 50 MPH carrying 300 lbs, so perhaps the demands on a bike are less than that of a car.
I'm also trying to decide whether I want to put the tires inside the tub, thus minimizing the width, but requiring some fiberglass or aluminum fenders to be made, or to put them outside of the tub giving me a wider trailer and potential more points for damage (except that the aluminum frame will be stronger than plastic).
I hate those posts the government likes to put in the middle of bike paths.