You can imagine all the fantasical Greek gunpowder weapons you like, but the sad fact is that even cannon were in their infancy when Constantinople fell. Although one might argue that a surviving or andvanced Byzantium would have used gunpowder weapons (as they did in that last siege) the idea of some kind of "wonder weapon" is far fetched. The Empire was broke and broken well before the final assault by the Ottoman armies. By all means let them have the "standard" gunpowder weapons if they survive long enough, but "flaming grape" and "medieval cluster bombs" are rather unlikely!