I posted this as part of another thread, but since it's a separate subject in itself, hope to get more responses as a separate thread.
One idea I had that I haven't seen anywhere: A recumbent tandem trike designed so that the passenger seat is above the driver's seat, rather than behind it. A double-decker tandem, in other words. That would have a smaller footprint, and also, if the driver was riding solo, the top seat could be adapted (with foldouts or by other means) to be a roof to protect from rain and sun and/or to carry cargo. Also, the vehicle would be high, which solves the problem of recumbents being too low to be seen by other drivers.
I have no idea if this is workable or not, I hope people here will tell me if it could work.
One person already pointed out two possible shortcomings: a high center of gravity making turns less stable, and the top rider dripping sweat onto the bottom rider. These don't seem like necessarily insurmountable problems to me, so would love to hear more opinions/ideas. Also, a link if you've actually seen a double decker recumbent on the net.