More european megacities.

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.
 
I'm pretty sure that if you can include the Randstad here, then you're going to have to define London's population as about 30 million. The distances involved between the four cities of the Randstad are easily equivalent to "annexing" a number of major population centres to London (Cambridge, Oxford, Milton Keynes, Chelmsford, Ipswich, Reading, etc) and are not that far from being able to add places such as Southampton, Coventry, Birmingham etc. If you're just including any area where a number of cities are linked by built-up areas in between then you can pretty much count 3/4 of England as one single population centre.
 
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