AFAIK, there are only three approaches to improving FE:

1. ICE thermodynamics
2. Vehicle weight
3. Vehicle aerodynamics

The Prius and Insight are shaped the way they are in pursuit of (3). Other shapes are possible as functional designs wrt FE, but a box will never be one of them. I don't see the Prius becoming more corolla like, but just the opposite. I drive the future; and luckily for me, I think the shape is beautiful.

R2-E2, 2G Prius.
Highway/City/Husband/Wife MPG: 56.5, as of 12/2005, 26K miles

Jac Nasser, Ford President: "We are planning to launch a hybrid version of
this car [P2000] within this year [1998]. We will also make FCEV available in
2004."