In 1169, Byzantine Emperor Manuel I Komnenos made an alliance with King Amalric I of Jerusalem to attack and conquer Egypt. The Fatimid Caliphate had been weakened and reduced to roughly modern-day Egypt I believe. Manuel wanted Egypt for it grain and wealth, and to keep it out of Nur ad-Din's hands.
IOTL the invasion failed due to a lack of cooperation between the Crusaders and Byzantines. But, if they had cooperated and the invasion been successful, what effects would there be?
* How much Fatimid territory would the Byzantines be willing to give up to the Crusaders? I imagine they would want to keep important areas like Cairo, Alexandria and Damietta.
* How would a successful invasion affect Nur ad-Din's plans? Would he still have sent Shirkuh to invade anyway?
* How long would Byzantine and Crusader rule have lasted in former Fatimid territories? Would the conquest only give Nur ad-Din more justification for his actions?
I'm thinking up another ATL scenario where the invasion succeeds, Myriokephalon turns out better for Manuel (he doesn't lose like he did OTL but the victory isn't crushing) and Manuel establishes a stronger Komnenian dynasty which seeks a policy of ruling over the Balkans, Anatolia and Egypt while keeping the Crusader States and Armenian Cilicia around as vassals - or at least until they can annex them peacefully.
It's probably just fanwank on my part though...