In Ottoman history the first singular devastating period of civil war to their righteous quest for glory was the defeat they suffered at the hands of Tamerland when he expanded his empire into the Anatolia. The defeat began a 12 year civil war between the sons of Sultan Bayezid. That period contains several possible POD that could have occurred but, following the reunification of the Ottomans a period of expansion followed lead by Mehemed II. It was marked by increasing sedetarianization by the Ottoman Court as it solidified power over a wide range of subjects-the most notable being the expansion of the Jassinaries.
Notable in opposition though there was the White Sheep Turks in Iran, Iraq, and Eastern Anatolia under Uzun Hassan from 1453-78. The two Turkic empires represented two interesting parallels. Notably the Ottomans who took legacy from the Seljuks and who were attempting to forcibly take out the nomadic aspects of Turkic life and the White Sheep who much more nomadic confederated oriented and took inspiration from the Mongols and Timurlane. On several occasions these two empire's clashed, notably Uzun threatening to once again destroy the Ottoman State. Though largely the two were evenly matched in their fights with the Ottomans relyin on their expert ability to Marshall military force and gunpowder tech. Meanwhile the White Sheeps used maneuverability and the terrain to force back the Ottomans.
Notably at this time the Venetians and White Turk conspire and launched joint attacks against the Ottomans, but their cooperation was sporadic. In 1473 occurred a series of battles between the two Turkic sides notably without promised Venetian aid. Therefore in the last of these battles Uzun's forces were delves rated by a concentrated Ottoman force via firepower.
What if instead the Venetians go on the attack against the Ottomans in Rumeli-Balkans, and for added measure the recently conquered Karananids and Trebizond areas revolt, an maybe even Hungarydoesnt have a dispute with Venice thus stretching the Ottomans forces and allowing for the Anti-Ottoman victory