Was Santiero already counted as such?
I assume you mean Santería? Because that's more syncretism of catholic traditions with native/african ones, and a deeper focus on saints and other figures outside the Trinity. Some Christian mysticism too, from what I understand.
This is already OTL: in fact, I'm surprised that many posters already offrered answers as hypotheticals when they
actually happened. Umbanda and Macumba are among the largest, but they are not the only ones. All across Latin America there are syncretist traditions, from the Maya to the Inca through many other peoples; in Perú and Bolivia indigenous people are becoming more open on the worship of Pachamama and the old Andean gods. Even in Argentina there is much syncretism: popular saints like Gauchito Gil, to small cults like San La Muerte, and in rural areas you can clearly hear how Christian belief has been shaped by Native and African traditions (Mandinga as the Devil, Tatá Dios as God, the Guaraní are virtually all Catholic, but they identify Tupã, their old main god with the Christian one.) The few surviving native ethnic groups in Argentina (like the Qom) are evangelical Christian, but their interpretation of the Gospel is filled with native references.
The problem with new religions in Latin America is that they will remain small cults at the fringes or syncretized native beliefs always skirting heresy because of the power of the Church in Colonial times, and even with the adoption of secular states. While the power varies from country to country, it is still a strong force that will not easily tolerate dissent, much less organized religions. To have an organized religion that breaks with Catholic/Christian dogma and is openly adopted by millions, you need it to be in a nation that has overthrown the weight of the Church. Even a revivalist Inca empire is difficult, since even the surviving Inca elites paid lip service to the Church to legitimize their power. But as they felt more confident in their independence, and the common people began to be more open in their worship of native gods and customs, there could be a point that they couldn't be considered Catholic anymore; perhaps not fully pagan, but a Christian derived religion like Mormonism.
Another option is for Black Africans to be more sucessful in creating their own states in Brazil. Quilombos (free black towns, sometimes having their own government and as large as cities) already existed. If they managed to stay independent, the religious consequences would be interesting.
Currently Brazil might have one of the largest religious diversities in the world, and certainly the largest in Latin America. There have been all sorts of religious denominations born there, from evangelical churches to the African traditions already mentioned to the Cult of Reason. It's not inconcieable that one of them could reach world religion status, but I'm not sure how to accomplish that.