Plenty of great sci-fi movies from the 21st century have deserved a sequel and not received one. Neill Blomkamp's District 9 merited District 10 after its open-ended finale, and Cloverfield should have been the next great monster franchise instead of lending its name to totally unrelated projects. Other times, a sequel has arrived, but many years after the original, as we've seen with the 28-year gap between Tron and Tron: Legacy , for example.
In each of those cases, the demand for a sequel has been driven mainly by narrative. Either the first movie gave some hint that the adventure would continue (in District 9 's case, Christopher promising to return and the opening of a "District 10"), or the fictional universe is so large that further installments feel like the obvious

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