Return to Pandora.
Why it is hard sci-fi
Soft. Cameron continues the Pandora project with new biology, the Metkayina sea clan and the tulkun, sentient cetacean equivalents with a verifiable language and culture. The films best worldbuilding is in the marine biology. Tulkun anatomy includes a real biological bioluminescent fluid called amrita that the human extractors target. The colonial-extraction premise is treated as a genuine industry with a working economy.
Science inside it
Marine xenobiology with a single high-value target compound, sentient non-human language and culture, partial-pressure breathing apparatus for hybrid Na vi-human use, and the whaling-vessel design as a working industrial platform. Cameron has form on this kind of detail and the film cashes it in.