Byzantine strength in Anatolia is at its absolute apogee in the 10th and 11th centuries, and will be regardless of whatever happens to Nicephorus Phocas and John Tzimiskes. The Byzantines have forced most of the Muslim states of modern Syria and northern Iraq into vassalage, and have destroyed the Arab rulers of Crete, Cilicia and Antioch. They won't be challenged in this dominance of the Middle East until the Seljuk Turks arrive in a century's time. As the Fatimids commented at the time- "the field is left open to her (the Roman Empire). She has been able to seize what was previously close to her, and to nurture ambitions that until recently would have been unthinkable".
So, in this context, here's a potential POD for you, if you are considering writing a "Bulgaria survives" TL.
969- Nicephorus Phocas discovers the plot to have him killed by John Tzimiskes. He orders the execution of Tzimiskes, and his co-conspirators, including the Empress Theophano. Nicephorus, who was deeply in love with Theophano, goes mad with grief.
970- Showing his customary lack of diplomatic ability, Phocas angers his ally Sviatoslav of Kiev, who has occupied Bulgaria, and holds Tsar Boris and his daughters hostage.
971- Phocas, faced with outright revolt, retires to a monastery (he was always threatening to do this in OTL), and a council of regency is set up for the child Emperor Basil II. The Byzantines bribe the Pechenegs to attack Russia, forcing Sviatoslav to turn north. In his absence, Boris and his daughters to escape to Constantinople, where one is married to young Basil.
980ish- Teenage Basil II leads Imperial troops into Bulgaria and occupies the country, defeating the Rus utterly, and driving them back across the Danube.
1020ish- Basil II dies, and civil war breaks out between his two sons, one of whom is in Bulgaria at the time of his father's death, the other is in Constantinople. The Constantinopolitan son eventually wins out, but he is forced to concentrate on Anatolia, to protect it from the Fatimids, and soon, the Turks. The Bulgarian brother meanwhile gains sovereignty peacefully over most of the Empire's Balkan territories, aside from Thrace, Macedonia, Greece and Epirus.
11th Century- Frequent intermarriages allow Byzantium and Bulgaria to become allies, though there are also dynastic conflicts. This breathing space allows Bulgaria to expand and flourish, controlling East-West trade from the Black Sea to the Adriatic.
12th Century- Maximal, perhaps implausible, Bulgarwank. Bulgaria rules modern Romania and Hungary, and the Tsar is recognised as the overlord of many Italian city states such as Venice.