#1, make it far in the future
#2, keep with the theme of an epic unbalance in the world that needs to be resolved, but with a different nation and a different way that the avatar hasn't been able to keep balance
#3, connect it indirectly with the first avatar series. perhaps the imbalance had its origins with a feeling of hatred towards the firenation for what it had done. this way, you open it up for an appearance by... you guessed it... Aang! it would make perfect sense, Aang could be the new Roku... and that way this series could close up all the loose ends of the original in a way that made sense to the new plot... ie, the new avatar must understand what happened after the end of the firenation war if he (or she, I suppose, but I don't know if I could get into a series with a female avatar, it'd probably be annoying to me) wishes to save the world this timearound
#5, definitely continue with the coming of age theme and somehow force the avatar to take on his responsibilities too early in life and keep with all that angst... and definitely continue with a conflicted bad guy charecter that must change to bring balance to the world
#6, keep it simple, please. leave the bending to the four elements and don't start introducing new powers or something. it would absolutely ruin the series... they've pushed the limit in this series with the bending of a person's energies, but it didn't cross the line. it's a very easy line to cross over, though, and I'd definitely fear they could end up doing it and if they did, avatar would no longer be distinct from series like dragonball or pokemon; always twistin up something new and exciting at the expense of the original concept.