We've had this conversation a few times and it tends to raise a few hackles! The cost of the chassis with suspension would raise the prices rather too high I think.
An Aero chassis Roadster (or for that matter a V6 Aero) would make sense with the current engine pushing out 280 bhp and 380Nm of torque. I know they would get immediate orders plus the benefit of increased volumes of production for the Aero chassis cars.
The 4/4 and the Plus 4 don't need an Aero chassis, IMHO, but they do need an updated chassis design and independent suspension all round. I can't see that it would increase costs significantly; Caterham seems to be able to compete with reasonable prices on their factory built cars. True the bodywork on a Morgan is much more sophisticated than a Caterham. The basic 1.6 model is more than £3,000 less than a 4/4 and all Caterham engines are tuned to a lesser or greater degree. For £27,995 you can have a factory built Supersport R with a 180 bhp Cosworth tuned 2.0 Duratec. Compare that to £29,500 for the base Plus 4 with a standard Ford engine. It is however true that not only does the Morgan come with fully formed mudguards but a hood and sidescreens as well so a direct comparison is not possible.
Now I am not saying I want a Caterham, it would not suit me at all, but what I am saying is that a small advance in chassis technology should not add much to the cost of a traditional style Morgan 4/4 or Plus 4. To change the chassis so that the axle was under instead of over the chassis would surely add very little to the cost.