4. Underwater Wildlife
This might seem a bit trivial, but it would go hand in hand with the confirmed marine environments we're going to see in the sequels. For me, some of Cameron's creatures above ground complemented the world beautifully and made possible some of the best scenes in the film, particularly the Thanator chase and Jake's initiation where he claims his Banshee. For this reason, I'm particularly excited to see what kinds of underwater animals Cameron will create for below Pandora's oceans. Whether or not they'll be predatory and what complications that'll create obviously remains to be seen, but it could make for some interesting situations. Maybe they'll be an underwater chase fleeing from a glow in the dark shark with three tails (or maybe something a bit less lame sounding than that). In any case, I'm excited.