It's certainly not impossible to have a Byzantine restoration post 1320, however it's going to require incredible bad and good luck for the Ottomans and Byzantines respectively. Which means that the story isn't going to be that entertaining...[/QU
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or you can have no Byzantine civil war. In otl Nikephoros II Paleologai managed to retake the Despotste of Epirus. This mean Byzantines still control Macedonia, Yannina, and Morea. Then have John paleologai die so no civil war breaks our and nikephoros remains in power. Then with the stability and no major war which decimates the Byzantines I can see the byzantines as a strong regional power but doubt they can become a major power. The civil war racked the Byzantines badly and in the process the Turks siezed most of Greece and Morea was devastated by the war. With thus the Byzzies lose valuable manpower reserves. So IMO In Nikephoros time most of the Greek cities such as Thessaly, Morea, Macedonia were all under Byzzi control