Ahmad Ibn Hanbal was a ninth century Muslim jurist and founder of the Hanbali school of Sunni Islamic thought. This school is central to all branches of orthodox Sunni Islam, it is the foundation for Wahhabism, and the central school of every modern Sunni terror organization today.
Unfortunate, because Ibn Hanbal was a really nice guy for his time period, but...
While he was practicing, another school of Islam was currently the most popular in the Abbasid caliphate- the Mu'tazila- which was arguably the exact opposite of the Hanbali school. It extolled the virtues of rationalism and science as the main methods of interpreting gods creation, believing that the Qu'ran was created by God through human hands and thus both imperfect and not above any other thing available in creation. I myself am a subscriber to this school.
However, during the ninth century, this school let its power go to its head. Caliph al-Ma'mun made the Maddhab the state school and have its jurists great power- this is when they started on a less lethal version of what I can only liken to the Reign of Terror in revolutionary enlightenment era France. They would imprison and in many cases kill orthodox Sunnis, and they did the same to Ibn Hanbal. However, Ma'mun put a stop to this madness and Hanbal was freed before he could be killed. (As a side note, this is why the Maddhab is not popular anymore: they abused their power and tried to force their views on everyone, and afterwards were barely believed to be a step above heretic. Rationalism would become the domain of the Ash'ariyya Maddhab from then on, which still believed rationalism was above the Quran but that the Quran had authority and was the divine word of God)
Here's the proposed point of divergence: What if a diseased rat bit him while he was in prison and infected him with a virulent disease that killed him very quickly? Before he had a chance to be freed, or soon afterwards, thus preventing the creation of the Hanbali Maddhab.
The butterflies will probably flap long before the present day, but the Hanbali Maddhab really only regained its prominence in the 19th-20th centuries, when Wahhabism was founded and Hasan al-Banna founded the Muslim Brotherhood.
I am a student, I do not know everything, so I propose the question here to you fine, more highly educated and experienced folks.