What's going to happen?

Here's my two cents:

Hakoda: Mortally wounded in a botched attempt to rescue the other water tribe and earth kingdom soldiers. Dies in Katara's arms when she's unable to heal him.

Sokka: Cut down in battle by a random fire blast from a fire nation soldier. Toph, insane with grief, goes on an earth-bending rampage, collapsing most of the Fire Lord's palace, killing herself and most of the palace's garrison in the process.

Suki commits ritual suicide to be with Sokka but unfortunately has to contend with Toph and Yue in the afterlife.

Zuko is finally reunited with his mother - right before Azula slips from the shadows and puts a knife in her back. Azula, now completely insane, blows herself up, taking Zuko with her.

Ozai, in the final battle with Aang, kills Appa and Momo with a single massive fire blast. Aang, insane with grief and rage at the death of his animal and human friends goes Avatar state and turns Ozai into a grease spot.

Mai, when she hears about Zuko's death, opens up her wrists in her prison cell. Ty Lee, hearing about Azula and Mai's deaths, hangs herself with her own hair braid.

Iroh becomes the new ruler of the Fire Nation.

Katara goes back to the south pole and assumes leadership of the southern water tribe.

Aang, a burnout after killing Ozai, seeks spiritual enlightenment. He becomes an advanced tantric yogin, meditating while standing on his head in the middle of a ring of smoldering fires, dwelling in charnel grounds, smeared with human ash and holding a skull bowl full of entrails.

What? Too dark?