Clearly day-long sustained highway speeds aren't going to happen any time soon given current technology, but I'm not sure a rooftop solar panel on a Leaf is completely uninteresting. If you could get, say 500 watts out of the thing, and you leave it sitting in a parking lot for six hours, that's maybe 10 miles you wouldn't have otherwise, particularly if where you are parked doesn't have any public charging, which where I live is just about everywhere.
My question is whether a feature like that be made available at reasonable cost. If it's a $500 option it would be appealing, $5000 fuggedaboutit. Also, can it be integrated in such a way as to not (further) dork up the appearance of the car.