I'd really like an epilogue. You know showing Aang and Katara with their air bending baby to show that the airbenders can come back. Zuko finds him mom, Toph makes up with her family, Sokka and Suki getting married,
I'm a sap for great epilogues.
Some kind of Spin off with a new avatar would be good along with that. I don't think they should just leave the Avatar world hanging