I don't think the idea is impractical, because I've driven many miles carrying a 10' x 4' x 18" plywood boat on top of a car that's smaller (yup,
smaller) than a Geo Metro (an 800cc 3 cylinder Daewoo Matiz):
The boat was made of 1/4" ply with a 3/8" bottom and had internal tanks, so a super-light teardrop body should weigh not much more - and the weight of the boat was only just noticeable and 'spirited' cornering was no problem (except perhaps for scaring other traffic...). If you built a roof-topper, I think you would have to carry 100% of your stuff in the car - except maybe bedding is light enough to stay 'up top'.
Aerodynamic drag is a whole other subject - that boat dropped the (indicated) top speed from 85mph to 77mph, which was better than I expected! But a teardrop 4' high would be a
lot more drag, so you'd probably pay in gas the cost of a trailer in one or two long journeys. Incidentally, aerodynamic stability was no problem at all with the boat on the roof - if anything, the car was more stable in crosswinds.
To make this idea work, I'd look at:
a) making the teardrop in two halves so that the top could be lowered over the bottom to reduce the height, and
b) it would be so light that it would be tempting to just lift it off the roof of the car and set it on the ground to use it - avoids all that ladder use.
Andrew