The hard part in this POD are keeping the Balkans not Italy. You need to stop the Slavic migration into the Balkans. I think the best way to do that would be to get rid of the Avarian conquest of the Hungarian plains. It would salo mean that the Langobards could stay in Pannovia. So what if the East Romans support the Gepid and the Gepids wins over the Langobard-Avar alliance, maybe even conquer the Langobards. The Gepids end up with a nice big state in Hungary-Transylvania and serve as roadblock for Slavic expansion into the Balkans. While other Germanic tribes may invade Italy, the conquest of the Lombards by the Gepids may keep Germanics out of Italy for a generation giving the East Romans time to cement their control over Italy.
When this are said Byzantium without having to figfht pointless wars in the Balkans, without the collapse of the Balkan economy following the Slavic invasion are going to be much stronger and richer.
There is a little problem with that scenario just as it was with OTL.
Even before the defeat of the Gepids the Lombards have entered Italy (in historic and archeological terms Italy starts in the Emona valley) and have penetraded Pannonia Prima entirely as well as most of Pannonia Valeria. Pannonia Savia was also mostly taken.
The Slavic raids across the Danube started with the Gothic war and there are strong indications that Slav raids have been spured by Goths. But very soon we see Slavs fighting both against Byzantines and fighting with the Byzantines against Goths. So a rather complex situation probably deriving from still very oportunistic and fractured mentality of the Slav warlords.
At the same time Gepids and Slavs were on quite good terms. There are no reported (though I admit sources are rare) battles between the two. So a surviving Gepid state probably wouldn't deteer Slav raids onto the Byzantium.
I think that a strong Langobard state would be a far better deterant.
A very important way for Byzantians to keep Italy and Balkans would be to not have the Gothic war.
After defeat of the Vandals it would have been better for Justinian to consolidate his position and leave conquest of Italy to one of his successors.
The Ostrogotic state was about to be hit by a wave of Slavic migrations and a strong Byzantie Empire could direct them into Gothic terriroty. It is very likely the goths would be severly weakened and fractured. At the same time slavs would establish a number of principalities which individualy could be dealt much easer than a solid gothic state was.
Another very important factor is, no plague which hit the mediteranium world very hard but from bone analasys it is shown areas north of the danube were either minor or non-existant.
In short for Byzantium to "keep" Italy and the Balkans it would need to play safe, with small leaps followed by times of consolidation. Also a Papacy surounded by schizmatic and pagan peoples would be much more closely connected to the Emperor.