You simply cannot improve on perfection.

You can weave in more relevant lessons about stuff, also about more current topics. Nothing about the old one is really out of date though so it'd just be additions here and there. You can even roll in plenty of Korra stuff, it's not that the series doesn't have anything to say it's just all over the place.

What any adaption certainly needs to do is start out with a list of lessons, not plot points. Change the plot all you want as is necessary for whatever medium, as long as the lesson aspect is intact people wouldn't mind, also keep the "reveal the world to the protagonists" vibe. E.g. Sokka doesn't need to learn his sexism lesson from Suki, it doesn't even need to be Sokka who learns that lesson, but that lesson needs to be taught. It doesn't even need to be about Avatar Aang, there's plenty of others. Earth is next, isn't it.