Paradox has announced a new DLC pack for Crusader Kings II called Sword of Islam, a mini-expansion that will let players control Muslim rulers. New mechanics will be added to make playing as a Muslim nation feel distinct from playing as a European power. You'll have to worry about Islam customs about marriage, holdings, titles, and laws.
Sword of Islam is going to be available in June for $9.99, which is an interesting price for Paradox. Most of their expansion packs run for $19.99 - $29.99, while their add-ons are usually under five bucks. This most likely means that it will be slightly larger than their usual DLC, but not quite as substantial as something like A House Divided.
A major patch is coming to Crusader Kings II right around the same time Sword of Islam comes out, fixing some bugs and tweaking combat, and adding a new area to the game map. This patch will be free, of course.