I heard we have a Suki, so naturally I have to say Suki/Sokka.
Zuko v Toph, burnt feet
Sokka/Toph, PIGGYBACK RIDES
Waterbenders in the fountain
I love you. XD Though why Toph would want piggyback rides when that means she can't see, I have no idea.
Jet/Zuko/Sokka, as a pairing or just being failbros together
Sokka and his four ladies, Suki/Yue/Toph/Ty Lee, assuming a Yue shows up. If not, just Sokka/Suki/Toph/Ty Lee
Toph and Aang, earthbending lessons with Aang sucking
Katara and Aang, waterbending lessons with Katara being pissed that Aang is better than her (might require a different fountain >> )
Some sort of commotion leading to the obligatory despair of Cabbage Man, and maybe even some angry Cabbage Man revenge. Or just Cabbage Man photobombing everyone
Toph, Zuko, Sokka, and Jet having a Manly Man competition.
SPOILER: Sokka losesZuko/Mai being their emo-lovey-dovey selves
Bad Guys vs. Good Guys
Men vs. Women
I know we don't have the right versions of some characters for some of those to be series-accurate, but I think they'll still be funny.
EDIT: One more.
Katara/Toph/Suki or Katara/Toph aka the girls of the GAang, either pairings or just general