There will never be any Arab Invasion mod.
It would be quite disturbing for muslim players and christians players alike to play a game in which hordes came out of the Arabian Peninsula, slaughtering everything in theyr path, burning famous libraries, killing christians and zoroastrians, etc. Its simply too disturbing, especially as it would mean to represent some of the companions of Mohamed in the game, etc... it would be scandalous for both muslims and christians alike.

For the christians, playing the Crusader era is not much of a thing, due to the fact that those happen around 1000 years after Christ, they contradict the christian doctrine, etc...

But when it cames to Arab Invasion, it is not the same thing