That might suit the Venetians better anyways, as an island would be much easier for them to protect, particularly for the more naval oriented Venetians, while still leaving the Byzantines themselves with vital control over the Bosporus.
The obvious problem being that the Byzantines would need a combination of military force and a lot of skilled diplomacy to keep the Venetians from making an attempt at taking or vassalizing what remains of the empire itself. They had done it before after all, when the Empire was in a far stronger position. The focus for the Emperors would then have to be to make it as unprofitable as possible, because without many years of recovery there is simply no way the Byzantines could defeat them at sea, or even on land for that matter beyond the walls of Constantinople itself.