I kinda want to see the Water Tribe Civil War happen. Get some bloodshed going on in there. Bigger the stakes the bigger the payoff will be in the end. Also, since this season is about spirits, we may theoretically see any and all the characters from the original series.
And a heads up for those who don't keep up with the writer's blogs and Comic-Con panels, we're eventually gonna see the first Avatar. His name is Wan and he lived ten thousand years before the events of LoK. We may even hear about other Avatars