More megacities might be an inadvertent result of domestication timelines like my own or Hueyuecoyotl's. With larger Native American populations and (possibly) less European emigration to the Americas, you might get more people in Europe moving into the cities during the 18th and 19th centuries and thus getting more large cities.
EDIT: Though there would also be increased emigration to South Africa, Australia, and quite possibly the less densely populated parts of Eastern Europe as well.
EDIT: Though there would also be increased emigration to South Africa, Australia, and quite possibly the less densely populated parts of Eastern Europe as well.