More a mind experiment then anything else, tough with the low requirements for HOTT2 I might actually do it.
James Cameron Avatar as inspiration for two Hordes of the things armies. Mainly the last battle in which the indigenous creatures aid the cause of the natives.
Now I am thinking 6mm, since that is what's on my mind at the moment, but there are many other options. Obviously the navi could come from 8-12mm fantasy miniatures, in those scales something one can get away with.

But this could perhaps work for other scales as well.
Now where I mainly was wondering at you guys idea's is the units and their appropriate rules, I could think of quite a few but perhaps you guys can figure out some other fitting combinations?
Humans
Airships, for VTOL's obviously, could also be dragon.
Artillery I dont think there are any in the movie, but a big gun would be appropriate.
Behemoths For their large power armors.
Shooters For any footsoldiers.
Magicians A bit of a stretch, but one of these could represent high tech gizmo's?
Na'Vi
Flying hero Jake sully, I originally thought dragon for him, but the dragon rules are written to represent cowardly megalomaniac super lizards.
Flyers obviously.
Riders again.
Behemoths I recall some pretty darn large creatures fighting on the side of Pandora.
Beasts Naturally.
Warband I think these represent best the fighting style of the Na'vi
Lurkers Indeed, beasties.
Sneakers I think this one might be nice for the further heroes of the Na'vi cause, springing clever attacks at the hart of the enemy? Else beasties again.
Clerics There was some mojo going on in the movie, and this can represent that if wanted.
So does anybody else have some cool suggestions for this? Personally I can really see this working, but perhaps I am wrong? Do point it out if so.