Lets say due to intelligence failures, Nikephoros I finds himself blundering into the decisive battle he was trying to avoid and is pincered by Harun al-Rashid in the resulting Battle.
Nikephoros I, Muhammad ibn Harun al-Rashid, and Harun al-Rashid are all killed in the Battle, the Byzantine Army is destroyed and Anatolia now lies open.
Abū'l-ʿAbbās ʿAbdallāh al-Ma'mūn ibn Hārūn al-Rashīd is now the Caliph.
So:
1. Who succeeds Nicky?
2. What does he do with the Bulgars on one side and al-Ma'mūn on the other?
3. al-Ma'mūn Can slice off what he wants, but does he go for Constantinople or just sticks to what he grab outside of Constantinople?
4. Does the loot prospects deter a Khurasan Revolt because they realize more profit to be had in going to Anatolia under al-Ma'mūn's banner and potentially getting good booty and new lands.
5. al-Ma'mūn initiated a lot reforms that never quite managed to complete before he croaked, could a longer time ruling enable critical reforms to go through and stick?