Note: This is not the joke about Katara marrying a powerful Bender. That would've been MUCH QUICKER, damnit.

From left to right:
Aang as Philip J. Fry, the hero and frequent saviour of the world;
Momo as Nibbler, cute animal sidekick;
Katara as Turanga Leela, the love interest and kick-ass warrior;
Sokka as Bender Bending Rodriguez, the comedy relief and occasional substance abuser;
Toph Bei Fong as Amy Wong, the rebellious rich kid and Other Girl;
Uncle Iroh as Lieutenant Kif Kroker, the wise and long-suffering advisor to:
Prince Zuko as Major General Webelo Zapp Brannigan, antagonist, alternate love-interest and Mr Bad Decisions.
Please imagine Appa in the background with a wocking great PLANET EXPRESS logo on his head-arrow, Ozai as Nixon, Azula as... I don't know, Morbo or someone.

Dr John Zoidberg is sad because I couldn't think of an Avatar alternate for him, but at least I didn't leave him out.

Happy Birthday to Tom - this is for you, and by God it nearly killed me. You better like it.

(It should be obvious to all by how badly they're drawn that I do not own Avatar, Futurama, or any of the characters therein).