Well this isn't exactly that wish list, but points the way on most counts, and a fair number were built.We already know that a relatively dirty aircraft with only moderate span and a relatively small prop isn't going to do well on low power.
Do we have an example of a clean ultralight with 35 feet or more of span that's low powered but swings a nice large prop at, say, 2,000 rpm? (And the prop must be a good one!) I think a lot of the problem is inefficiency in the propeller.
Cantilevered 31' wing, 43018 foil, 195 lb empty, 400 max
20 hp, Cuyuna 215
fairly lossy 2.25:1 multi-belt redrive
Prop 58 x 24, 2900 RPM