The #1 and #2 best selling "cars" by volume are trucks and trucks represent ~4 of the top 10 best selling cars overall. (1)
Musk has said that he has "this idea for a really advanced electric truck that has the performance of a sports car but actually more towing power and more carrying capacity than a gasoline or diesel truck of comparable size," Musk said.(2)
While this is probably several years out, as it would likely result in an all new wheel-base, etc I think that would be a hugely compelling product line if it can compete on the same stats w/towing / carrying capacity
(1) http://blogs.cars.com/kickingtires/2013/02/top-10-best-selli...
(2) http://www.autonews.com/apps/pbcs.dll/article?AID=/20130410/...