I think there is a mainstream doctrine for Wicca---but it is within the oath bound, line aged covens. One of the many reasons for lineage and oaths is to keep that doctrine undiluted and consistent.
What do you consider mainstream Wicca? Gardnerian? Dianic? Solitary certainly isn't mainstream, it's normally eclectic in nature. There are many different paths in Wicca. Different schools as well. Many of them do have their own doctrines. Some are very different as their belief systems are from different cultures.