Firstly, the Nouva 500 (1957-1975) had a folding canvas top only in the first few years. It was soon replaced by a solid roof with a canvas sun-roof (slightly larger than normal).
Since the 500 never had a real convertible version, I think what we see in these photos would be a real mistake. To my honest (yet biased) opinion, a much smarter move would have been reviving one of the 500 based roadsters of the 60's, the Bianchina or even the Vignale Gamine (by buying the brand from Ford, I'm sure the Americans would be happy to get rid of that).
That way Fiat would have created another niche model, created more buzz and remained true to the origins of the 500 - not just copy the Mini cabriolet.
My opinion, anyway.