Okay, here's a what-if that is partially inspired by the many maps by newer members that I have seen (most FH) which show Iran in the Arab league. Your challenge is as follows: with a POD after 800 (or 750 if you can think of nothing in the 9th century), make most of the population of Iran speak Arabic as their first language by 1900. They can still speak Farsi, but not as the native language. Bonus points if Farsi is dead or moribund.
Disclaimer: I am not anti-Persian.
Go.