Hannibal wasn't the leader of Carthage, mind you, so even if he had wun the war that wouldn't have meant he would have ruled a large empire.

About the Mongols, it seems to me highly unlikely that they would have been able to take western Europe if Ogadai Khan wouldn't have died. Europe's high density of cities (yes, back then already) as well as it's numerous rivers, islands and peninsulas would have made the manouvering Mongol armies needed pretty much impossible. If the Khan wouldn't have died by then, the Mongols would probably have remained a (admittedly, very serious) nuisance in eastern Europe.