I've heard military historians debate whether Hannibal really could have moved on Rome after Cannae. It seems as though he had secured some alliances with regional powers and Rome was vulnerable. But for whatever reason he didn't, which of course led to his final defeat at Zama.
How different would the world have been? Would we be living under an African empire? Of course the Gauls would have likely benefitted from a Carthaginian victory as well.