Rin: his 'revenge' obviously, as well learning to write/talk to his family/friends while in Australia maybe? The focus would be on harurin conversations for obvious reasons but some scenes with him talking with people like Sousuke, Gou, and Rei for example would be nice too. And making new friends with people on his swim team.

Haru: Getting more and more used to the competitive swimming world is a big one. I think you could something where he has some trouble adjusting to it at first but then he gets more used to it with the help of his new team (plus people like Rin and Makoto in the background). Other than that he, like Rin, could also probably learn how to talk to his friends now that he's a relatively good distance away from most of them.