What if Constantinople had never fallen to the Turks?

[TinCow]

You don't get it, in 1543, the Byzantine Empire was reduced to only Constantinople. The Ottomans already took the lands East and West of them, Bulgaria and Anatolia.

That's not quite right. Most of Morea (Peloponnese) was still under Byzantine control in 1453. Some of Constantine XI's advisers urged him to flee to that province to escape the fall of the city and continue the fight, but he refused.