The Kingdom of the Lombards held most of Italy by the mid 8th century, with the remainder (Sicily, Sardinia, the southern tips of mainland Italy, and the areas around Rome and Venice) held by the ERE.
The Lombards had gained quite a bit of influence over the Papacy and Rome over the course of the 750s-770s, as the ERE had its hands full fighting the Caliphate and the Bulgars, and the Papacy was falling out with the ERE over theology and church-governance issues. If the Lombards had succeeded in closing the deal and vassalizing Rome, they would have had a chance to roll on and take the rest of Italy from the ERE.
IOTL, this didn't happen because Charlemagne (who had previously be allied with the Lombards against his brother Carloman) invaded in support of the Papacy, leaving the North in Frankish hands, the center as the independant Papal States, and most of the South as a rump Lombard state.
A POD of Carloman surviving (rather than dying of a sudden illness in 771) would butterfly away the Frankish conquest of northern Lombardy neatly: the Franks would have been divided between two rival states, neither of which would be strong enough to challenge Lombardy, both of which more concerned with each other than with Italy, and one or the other of them allied with Lombardy (IOTL, Charlemange divorced his Lombard wife shortly before Carloman's death, and Carloman was probably in the process of concluding an alliance of his own with the Lombards when he died).