Serbians under Dušan expand much further into Greece, seizing most of it. The Tsar goes out of his way to learn Greek and to incorporate local elites, then marries a byzantine princess, claims the eastern roman crown, and lures elites in Constantinople over to his side with promises of lands and gold, as well as deliverance from the Ottoman threat.
He besieges the dilapidated place; his new allies on the inside unlock a gate and let his forces in. He declares a new Empire of the Romans and Serbs, and rules thereafter from Constantine's city. His successors, increasingly Greek in genetics and habitude, gain acceptance from the locals as their saviours.