Have a Byzantine Empire that successfully maintains control of the lands making up the Megali Idea, Magna Graecia, Thrace, Macedonia, North Epirus, Pontis only north of the Pontic mountains, Phaselis, Malta, and the Crimea. Also leave an allied/dependent independent Armenia to the east of Pontis.
Establish a dependency/vassal state/allied state Armenia, that would be a junior partner, but a friendly partner to Byzantium.
This leaves Byzantine Greece with enough land to be prominent, but leaving enough land for a significant Muslim Turkish power to take over the rest, and effectively cut off trade from the east. The capital would likely be a city on the trade routes like Constantinople was. Perhaps Beirut, Damascus, Aleppo, Antioch or even Jerusalem (though I doubt it). Antioch seems most likely to me, it's secure, has access to the Med, and is an easy location to control Asia minor from.
If the trade from the east is still cut off, then the age of exploration still occurs and France still eventually becomes a major European power like OTL.