Jim and his team of writers wrote the story/scripts for the rest of the franchise. However, upon finishing, it turned out they had enough content to make four more movies.
Actual content or fan service? I'm not sure what other stories absolutely needed to be told based on the first movie or if they had to create more subplots to justify newer movies.
No story 'absolutely needs to be told', but that doesn't mean there aren't good stories to tell. Here are a few things that I, as an avatard, would really love to see explored.
- Jake and Neytiri have children, the first true Na'vi/Human hybrids. Does the genetic mashup strengthen or hinder the children?
- The humans. Is there more than one RDA base? Are humans still on Pandora despite that one batch which packed up and went home (allegedly)? With the unobtanium supply possibly running low, what does that mean for life back on Earth?
- Other Na'vi tribes. We only saw a few of many. For all we know, the omaticaya clan could be the lowest form of Na'vi in terms of advancement. It'd be awesome if they were the equivalent of remote tribes in Africa and there are other sections of Pandora where the Na'vi are as advanced as humans were in the 1800s.
- Wildlife. Jim could make a fake Planet Earth like documentary of Pandora's ecosystem and I'd gladly watch it.
- Many humans stayed behind. Will Ewa transition all of them into their avatar bodies like Jake? What will their role be? The Na'vi now have access to all the tech left behind by the humans. When the humans inevitably come back, the Na'vi will be able to put up a much stronger fight.
- The remaining humans would speed up the advancement of the Na'vi culture at an alarming rate.
Jim has said several times that they are working to cut it down to keep it to just three more movies.
I hope so. If they release one a year - then Avatar will be like the introduction and the 3 sequels will be the actual Trilogy.
It's already been 6 years. The next three will look different to the first one due to the improvements in tech and the time passed.
Then the new three will all look similar due to all being made at the same time.
I think that's how it should play.
I'm pretty sure that's how it is going to go down. The first film with its easy story and proof of concept tech was a great way to introduce the world to the Na'vi and Pandora. James has now laid the foundation to pretty much make any trilogy he wants. It's really exciting.