One thing I always loved about Hetalia was the possibility for centuries-spanning angst between the characters. They all have such complicated histories with each other! Everyone has tried to kill everyone else at least once! And then they all get together and have convivial state dinners and joke awkwardly about it.

Also there is something particularly AusHun about ~roleplaying husband and wife~ as this great kinky thing between them. They would get an illicit thrill out of it.