In my ideal "Narses has a heart attack and Justinian learns to trust B." PoD, I could see this not being impossible. It would require two things.
1) Belisarius improves his manpower situation, perhaps he does this by entreating local Romans and Goths (probably the poorest folk) into becoming conscripts - and try and reduce his losses in typical troops by having mostly conscripts fortifying towns, with a core of proper soldiers that are training them to their standard.
2) With a better situation, and later no Narses (and as such no Narses to interpret orders in the worst way possible), Belisarius has more success in the Italian campaign.
3) With more success, there is no need for Belisarius to use the ruse to take Ravenna - and takes it in time for Persia, leaving a capable lieutenant in his place when summoned to fight Persia.
4) Success in Italy and Persia leads Justinian to send Belisarius West (to retake Africa, or invade Spain, you decide). - this leaves Belisarius far away when the Plague of Justinian hits.
5) The Plague hits, and Belisarius is summoned, and told to bring his army back east to protect the Empire.
6) Justinian dies, and the Imperial Family (including Germanus) die. Poof. The leading figure of authority in the city is Theodora, with Belisarius on the frontier. Whilst she hates Belisarius, having him as an ally right now would be good - and offers him joint rule of the Empire. Belisarius accepts, stating that he'll return to the city when the Plague has passed and the defences in the East are reorganised.
7) Plague passes, and Belisarius returns to find Theodora sickly, the last victim of the Plague, having held the Empire together domestically whilst Belisarius has held it together militarily from say... Belisaripolis (nickname for small coastal settlement in Syria that is kept carefully quarantined?) - and on her death is made Emperor Belisarius, Conqueror of Italy and Guardian of the Empire.
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TL;DR - you either have to have a mass death - like I've illustrated, or an appointed inheritance, which is unlikely to impossible. Otherwise it requires a usurpation, and I'm not sure Belisarius can achieve that from his meagre beginnings in the west.