IIRC, the armies of the East (which Belisarius was in command of at that point) had declared that they wouldn't accept any of Theodora's choices for Emperor unless the army consented. Theodora, as the wife of Justinian, can remarry and declare her new husband emperor, but he'd be lacking major support. Belisarius, on the other hand, seems to have been disinterested in the throne (see his refusal to be crowned Emperor of the West), but if the army proclaimed him Emperor at the same time Theodora attempted to banish him or strip him of his command, he might just take the purple.
Narses was a eunuch, and therefore could not be Emperor.
If Belisarius did become Emperor, I have a feeling it would be similar to Basil II's reign. He's a brilliant general, but I always got the feelin that he wasn't superb in diplomacy or the complex workings of the Rhomanion government. He'd probably end te Gothic Wars rather quickly, and might be able to stabile the Persian frontier. Beyond that, though, I don't think he'd be able to start a stable dynasty, because too many nobles and courtiers would see their opportunity in this strong but foolish (politically) Emperor.