A rhetorical question, I assume. But a good question. "The Passion" brought Catholics and evangelical Prostestants together, because it focused on the central event of Christ's life. He will lose a sizeable audience if he takes this rumored tact in his next movie.
If Mr. Gibson does the Fatima movie, it won't be for the purpose of bringing protestants and Catholics together. I don't think that was his aim, either, in The Passion of the Christ.
I think Mr. Gibson wants to make films about things he feels strongly about, and he's wealthy enough to do whatever he wants.
As a Catholic, I look forward to any films he makes about his beliefs.
Why would he lose a lot of audience? Do you know anything about Fatima??