In NA, good locations for Megacities (if you use existing population centers) would be in the New BosWash area, the Los Angeles Metropolitan Area, Mexico City, and likely either the Chicago or Toronto areas. Asia would likely see Mubai, Beijing, Singapore, Hong Kong, Shanghai, and Guangzhou. Africa might see the expansion of Lagos, Cairo, Kinshasha-Brazzaville, Greater Johannesburg, and (if you count the Mid East as part of Africa) Dubai, Jerusalem, and Mecca. South America would likely get Buenos Aires and Sao Paulo, plus maybe Bogota and Lima as well as some new cities. Europe, well, given the amount of metropolitan areas there, it's anybody's guess.
All these are suggestions, of course.