Şehzade Mustafa was the son of Suleiman the Magnificent, and the heir to succeed Suleiman, being popular amoung the Janissaries. However, in 1553, due to the manipulations of Hürrem and Rüstem Pasha, Suleiman had Mustafa killed.
So, what if Şehzade Mustafa had lived, and succeeded his father...