I'm still thinking about what if a fourth Abrahamic branch would spring up (the Cathars were always a possible good choice), and considering how different Mormonism is from Christianity already, what if Joseph Smith had decided to make it into a whole different religion? Sure all of the Abrahamic religions claim to be the legitimate successor to the previous faith (the old ones have only gotten corrupted, they say), but what if Mormonism had been more blatantly different from Christianity. You know, the name "Church of Jesus Christ and Latter-Day Saints" aside, it already is.
Hah, I just googled "fourth Abrahamic faith" and all of the links are about Mormonism.