WI: The Greeks manage to defeat the Crusaders of the 4th Crusade and the Duchy of Athens and the Principality of Achaea are never founded. Could Constantinople have been recovered earlier?
Balkanic Byzantium, most probably. Even if Crusaders fails, admittedly, the Byzantines would have some troubles but less compared to OTL.
Maybe, as consequence, not so powerful Bulgarians and Serbs eventually.
But one of the weirdeest side effects of 1204 was to prokove the reinforcement of byzantine Anatolia. Indeed, the Niceans were forced to streghten and redevellop a land that was considered more or less as a buffer zone.
You could have in the 1200's the same situation than in 1300's : a Byzantine Empire as a balkanic, stricto sensu, power.