The obvious answer would be the victory of the pro-Roman factions in the city, which were mostly based on the rejection of a continental political influence : but these were tied to the existence of a relatively strong continental power (namely, Carolingia). Not only this significantly depolarized the inner politics of the early republic, but the lack of a strong adriatic rival made Dalmatian coast an after-tought in Byzantine geopolitics, which allowed South Slavic polities to gradually take on the region (partially out of a rear-alliance against Bulgars), which allowed Venice to pull a Carthage (by becoming the "natural" Roman-issued regional protector).
Maintaining Francia in one piece, or having a strong late Carolingian/post-Carolingian Italy would be really hard changes to get
(altough the former much more than the latter), but a less chaotic post-Carolingia situation AND a lesser Bulgarian threat in Balkans (or, possibly, a stronger Bulgaria that would appear as an Adriatic rival, but this is less likely to work out IMO) could do the trick, for a while.