Why is it impossible? The only dodgy film that he's ever done is Piranha II, and that was his debut film. Titanic was aimed at a different audience than Aliens/Terminator, hence why it constantly gets a backlash from the geeks on the internet. With Avatar, Cameron is back in his comfort zone of sci-fi fantasy, I really can't see it being a dud.
Saying that Cameron has lost his form purely because of the time frame between Aliens and Avatar is stupid, not to mention baseless. It's like saying Hitchcock lost his form once he left the UK and started making films in the US without having seen any of his US work...
I really wish people would wait and watch the film before slating it, because until then it's all biased opinion based on either a 3 minute trailer or the 16 minute preview, which hardly gives an accurate representation of the full 2 hours+ film. If after you've watched it you hate it, fair enough, but at least then you'll have watched the damn thing.