Lisa - this is a great dress and I can see Jenn in it, which is such a good thing as Jenn is the master of great black summer dresses.
For casual dresses my favourite toppers are denim jackets, leather moto or blazer, bomber jacket or anorak. I have very few cardigans, but I can see the right one working with this dress. I would have thought that this dress would look great with a topper because that would add more structure.
So glad you found a great summer dress. If this dress is beautifully made in great fabric that is easy to launder my bet is that you will gravitate to this and get heaps of wear.