I presume, when it comes to making ultra-light planes that are as big as the standard parkjet your options do get a bit tight. I'm imagining profile planes with small batteries.
I am wondering about simply building tiny planes - they won't stand a chance in wind, but they won't need a lot of space. My Hubsan H107C quad uses these minuscule brushed motors that are less than a CM in diameter, and you could power them for a good ten mins on a 650MaH 1S. Years ago I had this clumsy little bipe called an X-Twin. It had hardly any control, pitch was controlled with speed, yaw was differential thrust and roll was held by dihedral. The big wad of a nose made it crash-proof, but my point is that it was the floatiest thing and could cruise around at under 10mph - I would fly it around that graveyard in front of my house, which is otherwise rotor-wing territory. I imagine if you scaled down your Horten Ho-7 to a 20" wingspan you'd have a garden flyer like the X-twin but with a lot more control and grace.
BRB - Building another nosecone.