Thing is, other people have tried to do this. All of their results have been terrible. Same as most attempts to do anything else in d20 that isn't D&D/Pathfinder. Just look up Naruto d20.
My question for you is, what is this for? Do you actually want to play an Avatar campaign at the table, or is this just a thought exercise?
If it's the latter, go ahead and do both. Maybe use the time modifying a base class to try out a couple ideas for the from scratch class. Play a few test sessions with each and use what you learn to make even better stuff in the future.
If it's the former then the effort that goes into building a class for d20 might be a bit much for the pay-off in actual play. Especially since you're likely going to break the game with it, which leads to the need to fix it. That time spent fixing the class and the game could go to actual prep. Is that something that you're willing to put up with, including the game derailing, retconning events, retconning character sheets (telling the player that their super optimal build choice back in level 2 is now nerfed), and dealing with irate players who probably don't want to deal with all that crap?
Instead, if you want to play an Avatar game then I'd suggest that you try out
Avatar World. It works out of the box.
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