Okay. More ideas!:
Ty Lee, at Kyoshi Island, finds out that their visitor is Princess Azula. She immediately bows before the Princess. Azula tells her her plan to become Fire Lord Azula, and asks Ty Lee if she would like to join. Ty Lee tells her that she doesn't know, and Azula tells her she has a week to make up her mind. By the end of the week, if Ty Lee decided to join the group, she must be at the Fire Nation Palace.
Azula (somehow) gets into the Fire Nation Palace and talks to Mai about her plan. (Idea: Azula acts like a peasant who desperately needs to talk to the Fire Lord, but is obviously redirected to the Fire Lady, Mai.)
After the group's vacation, everyone goes their own ways: Zuko back to the Fire Nation, Sokka and Suki to Kyoshi Island, Toph to [someplace in the Earth Kingdom], and Aang and Katara to (either the Water Tribe or the Air Temple).
When Suki and Sokka arrive in Kyoshi Island, Ty Lee tells them about Azula. (Up to you, nalasimba83, if you want Mai to tell Zuko. I'd say not, but it's up to you.
)
Toph, Aang, and Katara are oblivious to Azula's threat. But [someone] sends them a message about it. Katara is angry at the news that Azula isn't any better. Toph, not being able to read, throws the paper to the ground in anger. [Someone (OC, maybe?)] reads the letter out loud. Toph's anger turns to shock. (You can each add your own characters' thoughts about the threat.)
(That's all I can think of. I don't want to control your characters, so I'm leaving the ideas to you two now, until there's some way I can figure out something else to do.
)