POD around 1250.
Mongols conquer much like OTL, but stop just short of subjugating the bulgars. The Mongol empire collapses quicker and into weaker successer states.
Second Bulgar empire expands into the power vacuum, including most or all of Pannonia, west to the Adriatic and east to the Dnieper. Develop a respectable navy.
Turks migrate into Anatolia weakening Byzantium. Bulgar empire finds some reason to claim the title as rightful successer to the eastern roman empire. Seize and hold all the Greek Balkans and some of the islands, maybe even including Sicily, Cyprus, maybe even Jerusalem.
Over next few centuries conquer Northwest to the Danube, up to or a bit past Vienna. Austria south of the Danube even now gas a South slav population.
Conquer up the Dnieper and Don basins.
Maybe grab parts or all of Moravia, Czechia, Poland , Anatolia, or Russia. Most of those would be hard but not impossible. getting all would be very hard. The Volga basin seems the most plausible, fellow orthodox Slavs, and historic territory of the Volga bulgars.
North Russian, central asian, and Caucasian polities as tributaries. Novgorod, Georgia, Siber, etc. Some might later be directly absorbed into the empire.
The remaining Slavs would be the catholic Lusats, Sorbs, Czezhs, Silesians, Poles mostly. If the germans behave as OTL those Slavs might be overwhelmed.