I'd go with all seven. It might be complicated, but it fits a wide variety of cultures and styles.Another question with this kind of thing is which elements to make available. The classical four, obviously, but aether? metal? wood?
True, but then you don’t really lean into the theme of ATLA all that much. The goal today is not to be as simple as possible.Edit: You could simplify it just by saying to choose a damage type (fire, cold, force, magical bludgeoning, whatever) and then having your powers be things like "you use your element to make a strike of whatever type, and it inflicts X damage of the chosen type" rather than have a big description for each element.
Sure, OK. But, from what little I know of ATLA (got as far as meeting Toph and then just kind of forgot to watch the rest of the show--I've never really been into that genre), each element was very discrete--you didn't get air benders who could use fire. Which means that you're effectively creating four or seven lists of abilities, at four different levels, for a single archetype. Which would take up a lot of room. (Of course, you could just create that many separate archetypes, in the way that pyromancer is an archetype for wizards, with presumably aquamancer, aeromancer, and geo/terramancer still to come.)True, but then you don’t really lean into the theme of ATLA all that much. The goal today is not to be as simple as possible.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.