Avatar Last Airbender game
All I can really say is that d20 (and especially Pathfinder) is overly complicated in ways I can't even begin to convey. I have cursory knowledge of Deadlands Classic, Savage Worlds, FATE (I think, it's been a while and I'm not sure I've connected the correct name to the idea), World of Darkness, Exalted, and D&D 4E, with advanced knowledge of Pathfinder and Marvel Superheroes. With that in mind, I have never seen a system more awful than Pathfinder, and my understand of D&D 3.5 is that it's not much different. I realize it makes for a great starting point for tabletop RPGs when you've already got the characters designed for you, and the fact that all the rules are available at d20pfsrd is super-convenient, but beyond that it just isn't worth putting your time into.
Among my experiences, though I am rather biased on this one, I think the best system--mechanically--for putting something like this together would be Marvel Superheroes (probably Basic Set). Everything you need for it is available on classicmarvelforever, and the learning curve on it is practically nonexistent. This works well from a mechanical standpoint, though it would require some slight changes to character creation (namely, primary stats go down while power ranks go up).
However, Avatar: The Last Airbender is very story-based, and therefore should probably be used with a story-based system. For this reason, more story-based systems like Exalted or Savage Worlds would help out a lot. While Exalted is explicitly too high-level and has too many weird setting-specific rules, Savage Worlds has some complications as well. Savage Worlds does, however, have Shane Hensley's Savage Worlds Super Powers Companion, which helps. Though that one also has some mechanical discrepancies that need to be fixed. Namely, the rule on power limits probably needs to be scrapped.
TLDR: No matter what you do, you'll probably need to use a little homebrewing to get things right. That said, your current system experience is a terrible kicking-off point, as Pathfinder is too carefully-balanced to go tossing in something like Bending, and 3.5 is also far too complicated to go messing around with stuff like that. Use something simpler and more story-based, like Savage Worlds. Learning a new system might not be fun (I'm working to learn Savage Worlds right now, actually), but it will definitely be worth it.