A good start would be preventing the overthrow of the Shia Fatimids in Egypt and north Africa. If we assume Egypt stays Shia, that may tip the balance.
The other factor is support for the Abbasid revolution, which initially had a Shia component. Let's say they openly come out as Shia instead of Sunni. That changes everything.
Another scenario is that Safavid Iran does better historically, and manages to convert more people to Shia Islam.
A final idea is that the Ummayads never come to power, and the ahl al beyt rule instead. Let's say Ali does not agree to peace with the upstart Umayyads in Syria but goes on to totally defeat them.
The Abbasids were never confessionay or in terms of aqeedah of the Shi’ah. There was some support from Alids in the east at the time, but the Abbasids were betrayed by these factions fairly soon after the battle of Zab. The Abbasid as a whole represented vicious persecution of Shi’i communities and repression of their imams and leaders by way of war and other similar tactics.
Shi’i also became targets of not only Abbasid attacks, but foes of the Mu’tazilah factions in the Abbasid court during the Mihnah. Most especially, those Shi’i who denied the Quran or those Shi’i labelled as Bataaniyyah or more commonly meaning Is’maili Shi’i.
There was also large amounts of rebellions against Abbasid authorities and counter movements that were either Shi’i or influenced by them. Some of the more famous being the Zanj rebellion, Qarmatian menace, Shumaytiyya, etc,,, numerous.
The Banu Umayya were not upstarts against Ali and the so called Iraqi faction. Mu’awiyah was commanding mostly the entirety of the Muslim forces in Syria and Arab Anatolia and by the final confrontation, signed pacts with Amr al-As, the emir of Egypt. Further, Mu’awiyah represented a certain Uthmanite legitimist and continuation and was not an easy foe. Suspect upon Ali of assassination of Uthman existed and his refusal to fully investigate the issue made him many foes, including Ayesha and others.
Regarding Ali defeating Mu’awiyah, there is likely a reason Ali signed the treaty. It is often reported that the forces of Mu’awiyah and Amr al-As held the advantage and initiative. Ali’s gamble for peace was to ensure he keeps his position whilst not endangering his persona. In the end, it is clear, Mu’awiyah and Amr al-As forced Ali back politically and if war is continued, Ali is likely to be outmanueverrd militarily for his perceived role in the death of Uthman Ibn Affan, or the role of many of his supporters who were present in his camp.