I haven?t really witnessed the development in the relationship between Aang and Katara, but it ends with them having some clich? moments at the Northern Water Tribe. They?ve a scene together right before the battle starts, and after the battle they have a nice hug. Not really spectacular. But I think their relationship is still at the beginning of the whole development, and that it really comes to an end in the sequel(s). They?ve mainly focused on the relationship between Sokka and the princess.
The Fire Nation rhino?s are in! Along with the Fire Nation tanks, they participated in the battle of the north. The soldiers on the backs of the rhinos where filmed at the same (green screen) department where they?ve filmed the gang riding on Appa?s back, so I haven?t been able to see any of these creatures. They?ll be added in post-production. No other animals as far as I know.
I?ve been part of the ?Ancient Firebenders Attack? sequence. We?ve filmed Firebenders on rhino?s and tanks as they destroyed a village, and a part where they set foot at the Southern Water Tribe the first time. The only thing obvious in this whole part is, the Firebenders wearing different armor.
The relationship between Zuko and his uncle is really like: he?s the immature, stubborn, ignorant, furious child, being looked after by his calm, patient uncle who seems to be the only one who can temper him.
Yes there were Fire Nation ship attacks. One, of course, at the battle of north, where the Fire Nation armada destroys the high walls and surrounding icebergs. There?s also a part where Appa flies over some Fire Nation ships, avoiding fire balls. I don?t know whether I?ve mentioned it, or not, but that was also something I?ve seen in the test screening of the trailer.