Where should James Cameron take Avatar next?

As Avatar's barnstorming success at the global box office – where it recently became the fastest film of all time to reach the $1bn (£627m) mark – continues with little sign of letting up, it seems remarkable that as recently as a few weeks ago some critics were claiming the project was set to flop.

How they must be wiping the egg from their faces now. Watching the film for the third time at the weekend, it struck me that James Cameron's movie already has that classic Hollywood blockbuster feel – remarkable for a feature that's so reliant on technology and, as Peter Bradshaw pointed out, a freakishly bizarre storyline.

This is going to be a film that kids and adults alike will remember 2009 (and 2010) by, and there aren't too many movies that you can say that about. For me, it's a feature that – like the original Star Wars trilogy, ET and Indiana Jones before it – causes the viewer to unfurrow his or her brow and wrap themselves up in sheer cinematic indulgence. Naysayers be damned; this is a flick that's far too much fun to get too po-faced about.

For those of you who've not seen the film and would like to, I can only suggest that you turn away now. For in discussing the planned sequels, it's going to be necessary to mention the way the movie ended.

Cameron has been talking about Avatar as a three-movie project since as far back as 2006. And given the incredible amount of technical groundwork put in by Weta Digital as well as Industrial Light and Magic to bring the spectacular world of Pandora to the screen, the invention of an entire language for the Na'vi to speak, and the fact that the movie is left reasonably open-ended, followups would make a certain sense.

"[There will be] possible sequels if it does well; if it tanks, no," said Cameron of Avatar three years ago, adding that he would probably shoot any followups back-to-back.

"I have a trilogy-scaled arc of story right now, but I haven't put any serious work into writing a script," he told MTV last month. Both movies would follow Avatar's lead characters, Jake Sully (Sam Worthington) and Neytiri (Zoe Saldana), and pick up where Avatar left off, on Pandora in the year 2154.

Beyond that, Cameron has been keeping reasonably tight-lipped about the plot details of the two sequels, other than to remind us that Pandora is not the only moon in its planetary system.

"The planet in Pandora's sky is called Polyphemus and it's the primary for a system of moons, just like in our solar system, where Jupiter has 50-some moons," he told reporters in LA last month. "We have some story ideas for how to branch out into other moons of the Polyphemus and Alpha Centauri A system. "

Cameron has a track record for making excellent sequels, having arguably bettered his own Terminator, from 1984, with 1991's T2: Judgment Day, and achieved the remarkable feat of taking on Ridley Scott's 1979 slasher-in-space classic Alien with the more action-adventure oriented followup Aliens in 1986. However, the concept of back-to-back sequels to continue the story of a successful origins movie does have the awful ring of The Matrix to it. And even more than the Wachowski brothers' film, Avatar's denouement is a little too open-ended: Cameron could go literally anywhere from here.

The obvious storyline for part two would be the return of the expelled humans, following a sort of The Empire Strikes Back blueprint. But screenwriters might struggle to find a worthy arc to mirror the highly enjoyable one-two combination of Jake's arrival on Pandora and subsequent induction into Na'avi culture. And without that, the movie risks becoming one long battle sequence. Furthermore, one imagines that the extra-Pandorans would simply nuke the Tree of Souls from space, rendering the moon's indigenous defences redundant. Cameron may want to avoid falling into that particular plot hole.

It might be best, then, to ignore the humans (at least until part three) and open up the possibility of a new threat, from a more defeatable enemy. But that runs the risk of looking like something of a damp squib when compared with Avatar's epic struggle for Na'vi survival in the face of apparently overwhelming odds. And it's arguable that the first movie required the presence of humans to ground it in something approaching reality – those who dismiss Avatar as Thundercats in space and guffaw at its noble savage stereotypes would no doubt have a field day with an all-Na'vi storyline.

To be honest, I'm a little stumped, which is where you come in. Imagine you've been given the job of coming up with the plotlines for Avatar 2 (and part three if you're feeling imaginative). Where would you take the adventure next? Or should Cameron just quit while he's ahead?