For the Franks during Charlemagne's reign and after, the threats of Avar and Moravia were prevalent. Marches in the east were already located at the far end of Charlemagne's empire; duke of Lower Pannonia (march) Ljudevit's rebellion in 819 against Louis the Pious, merely five years after Charlemagne, was evident for the fragility of Frankish governance on that end, even though the Avars had been effectively defeated. Ljudevit defeated duke Borna of Dalmatia. Possibly unknown to the Franks at the time, the Bulgar Khan Krum contributed to the fall of the Avar Khangate from its eastern territories, not to mention Krum's leadership soundly defeated the Eastern Roman troops and made a skull wine cup out of Emperor Nikephorus I in Pliska 811. Krum's successor Omurtag contacted the court of Louis the Pious -- it was him again -- in regards to the Slavic tribes not adjusting to Bulgar's governance. Skirmishes between Louis' and Omurtag's troops broke out.
If the Franks had advanced during Charlemagne's time, they would have faced foreignly the advancing westward Moravians in modern day Slovakia and southern Poland, the weakening Avars in Hungary and the rising Bulgar Khanate in Romania and Bulgaria, and domestically Ljudevit was possibly only loyal on the surface to the Frankish court. If the Franks had advanced, the diplomatic and military campaign would have been grand in preparation and influence. Maybe a crusade-esque expedition would have been needed. Pope Leo III was mutually supportive of Charlemagne. Maybe dukes Borna, Ljudevit or other nobles could lead an army northward to attack the weakening Avars while Krum was defeating Avars from the east. Given that duke Baldric of Friuli expelled Lujudevit with military force but was later unsuccessful in 820s against the Bulgars under Omurtag, the Bulgars would have won more territories and Slavs under the Avars than the Franks. For the successful Frankish campaign during Charlemagne's --- and hence Krum's reign --- in an ATL, the Bulgars needed to win less.
However, that ATL would be going in a circle because Krum's leadership was capable of defeating Eastern Roman troops under the Emperor Nikephorus' personal leadership. Why would Krum's leadership become suddenly less effective against the Avars when the Frankish nobles were around? Maybe Baldric and Borna could be very smart to take the low hanging fruit of territories and Slavs when Krum's leadership would have done most of the campaigning.