If we go by regions that are yet to host a world cup
East and South-East Asia(barring Korea and Japan), North/Central Africa, Australia, Oceania, South Asia, Antartica

baus

, still a lot of regions to cover. With FIFA looking to expand and make more money than ever, dont be surprised if no European country hosts a WC for another maybe 4-5 editions atleast.
Russia covered Eastern Europe and Qatar covers the middle-east(for now)
FIFA's target was always USA this time round. Population of near 400 million and richest economy in the world. Its all about dat market economics. Mexico and Canada simply happened to join the bandwagon because they would benefit from this even if its just a handful of games.
Next stop is 1.4 billion strong China.