I've participated in a group much like this concept - it calls itself the "Bring Your Own Book Club" - and it can offer a really interesting sort of discussion as people can consider the varying ways that the theme manifests in each book. It does require people to speak a little more eloquently about the book they read, because there's no guarantee anyone else in the group has read it, so they need more information about what's going on in the book. The BYO Book Club has a sort of survey/template for responding each month, that prompts for summary info on the book, but also asks some questions related to the theme.
That said, I tend to only participate in this other group when a book I'm reading anyway happens to coincide with the month's theme. BUT if we get a list of themes well in advance, perhaps I (and others) can tweak our planned reading to coincide...
