Chrysler engineers could have simply taken the gasoline tank and engine out of a Fiat 500 and put in electric motors and a battery pack wherever they'd fit. Job done. Costs saved. At least some people would still buy it just because it's cute and it runs on batteries.
Instead, they did a lot of work. First, they went out and spent time -- as in, basically moved in with -- a whole bunch of Mini E electric car owners to find out how they used their cars and what they really wanted in a small plug-in car.
With that knowledge they set to work creating a plug-in version of the 500 that would preserve the fun of the original. (It turns out electric car owners really don't want to drive golf carts) That's not easy when you've got a battery pack that adds about 600 pounds.