I actually love the idea of a Hybrid convertible. No one has done it yet. (I guess the ever shrinking convertible market doesn't really help)
I drove the Prius C last year and thought it felt really cheap. Although quite comfortable.
But a 50MPG convertible seems like a good idea.
Although there is already the VW Beetle diesel. A much more solid feeling car than the Prius C.
And much quicker too. Plus it does get over 50MPG already, on the freeway.
Still, this could be interesting...