I'd go so far as to say bender isn't just a class.
Benders are unique. You can't learn to bend if you don't have the ability to in the first place.
In that respect, bender could be a template, with or without any modifiers, that allows the user to replace a number of powers at certain levels for bending powers. (I thought for a split second about races, but that'd complicate things needlessly since you'd need some form of bender for every race out there, since 'bender' is just an addition to your race)
Although, somehow I think some kind of penalty should be included for this to be fair. Strictly spoken, without looking at game balance, bending is just something people can do aside from whatever else they can do.
However, if a character spends much time learning to bend (which you can assume a bender does), this should be reflected in the rules.
It could be a reduction in skills, feats, or limitations on powers gained or the acquisition of class features (and by 'limitations' I mostly mean that a character's level would count as less for purposes of acquiring powers and/or class features for their classes. Effectively, they learn the traits of their classes slower as they have to spend time and energy and experience learning to become a powerful bender).
Even though it's now not a class, there's still the option of creating Paragon Paths and possibly Epic Destinies for benders, such as a Lightning Bender (for those already skilled in fire bending) and Blood Bender (for those skilled in water bending) and perhaps Metal Bender (for those skilled in earth bending). Or you can just take paragon paths from your normal class rather than develop your bender abilities with added features.
I also don't think it would do the benders much justice to copy paste mechanics and powers to create them. Honestly, if you want to play an Avatar game, spellcasters aren't going to be present, for one. Bending is unique in that it technically doesn't have any predefined powers or moves. Predefined powers and moves can be made if no proper or balanced alternatives can be found, but using power acquisition such as normal classes do is strange, at the very least. It would be strange to find earth benders that could shove a building aside like it was nothing, yet wouldn't be able to send some earth spikes in the enemy's direction. (This goes against what I said of bender templates and power acquisition and such, but I'm just spouting the ideas that popped into my head as I read the posts in the topic and such...)
But that's my view on things.