For me the optimal co-op would one that met in two sessions, every other week. I'd like academic/arts classes in the morning and play/park/games in the afternoon. I wouldn't care if it was/wasn't religiously affiliated as long as there wasn't a required SOF.
The reason I'd want one day every other week is because we get nothing done when we have half day activities anyway, so might as well only lose one day instead of two.
On the off weeks there could be low-key, optional evening meet-up type activities, so that the dads and other non-homeschooled siblings could participate: things like skating (roller/ice), games at a park (soccer, b-ball, baseball, hockey etc.) that don't require a lot of conversation in order to participate, bowling, group bike rides....