The classic filling for zucchini flowers seems to be a mixture of ricotta and herbs, frequently mint, maybe with some parmesan mixed in. (cookandbemerry.com)
I made this today and added zucchini flowers on the top as a garnish and it was amazing! (smittenkitchen.com)
I didn't use zucchini flowers, and I had to use dry basil, but everything else was the same. (epicurious.com)