I actually never saw Shamanism as culturally specific- more of a sort of miscellaneous category representing minor animistic religions such as those of the Aborigines, Maori and Amerindians. The big problem, of course, is that Oceania in general is hosed (and they sure don't need it, living in that big ocean of theirs). Perhaps Shamanism could be split into Aboriginal, Amerindian and Polynesian varieties, much like how Ngaiism, Voodoo and Yoruba even the odds for Africa? A form of this is already in place for Druidism (technically an inaccurate name, but probably a better one than "Celtic Mythology") and Tengriism.
This might also change depending on how
my proposals for cultural evolution pan out; cultures would be far more closely linked to each other and some "ancestrally independent" civs would get their cultures from the start, like Persia (a reserved seat to get Zoroastrianism), China (Confucianism and Taoism), Mongolia (Tengriism), India (Buddhism, Hinduism and Sikhism, though the last was not addressed in my proposal since it was created before Sihkism was added), Ceylon, that fictitious Southeast Asian country, Japan (Shinto), Korea, Nubia and Polynesia (obviously, they'd get a reservation for any future Polynesian or Maori religion), with others serving as "progenitors" to the other civs, such as the Proto Indo Europeans, Semites and Clovis Culture.