Watch any popular James Cameron movie and you’ll notice a pattern. HI movies aren’t just about spectacular special effects although there re plenty in movies like “Avatar” and “Terminator 2.”
While most directors would just be happy with more explosions, gunfire, and action, this is never the formula for a good story. Just look at bad movies like “The 355” or “Don’t Worry Darling” to see how special effects and fancy settings can’t substitute for storytelling.
What James Cameron does is offer a theme and that theme provides the foundation for every dilemma every character must face. The “Avatar” movies are all about being unified with nature, so it’s no surprise that the villains (the humans) are always portrayed as indifferent to nature, wiping out forests and killing intelligent animals just for a minor resource while letting the carcass rot.
The dilemma the characters must face is whether to respect nature or abuse it. That dilemma forces every character to take action but their actions are always grounded in this simple dilemma: do they respect nature or do they dismiss it?
This dilemma occurs in a subtle way when the hero (Jake and his family) retreat from the forest to the ocean where they take refuge with the ocean people. Because the hero and his family are forest people, they must learn the ways of the ocean people, which causes friction among the younger ocean people who see the forest people as aliens.
This subtle conflict reinforces the idea of respecting nature or dismissing it, which goes along with the other problems of the humans killing intelligent whales for their precious fluid that stops aging.
With a strong theme, your story has a clear direction to you (as the writer) and to the audience (who may not even be aware how it unifies the story). Without a strong theme underlying your story, you risk just adding more mindless action that makes no sense and just creates a chaotic story instead of an emotionally satisfying one.