I'm starting to think that ATLA was a fluke to be honest, like you said, everything that got made after the original series was mediocre at best. I'd even dare to say that if the original show lasted for one or two more seasons it would've made ATLA overall worse, the cracks were already showing by the later episodes, that fucking asspull with the Lion Turtle giving Aang a new power so he doesn't need to kill Ozai, pretty much tells that the show (and probably the franchise as a whole) was already running out of steam.
I'd call it mediocre by todays standards, good by the standards of a lot of 90s shows.
Animation before this was basically either a super hero show, a kids WB action show (there were some good ones), or stuff like Dexters lab.
Its the same thing with Tartakovsky making Samurai Jack- action based and plot driven animated shows were kind of a rarity and starting to make a comeback with the popularity of anime. Most of the action shows just didn't have overarching mega plots. Avatar did, and managed to really world build well
In that sense, avatar wound up being anime-esque, much like Teen Titans, imo, but with a wider world, and one that did try to push a wider story.
If you actually look at the story, honestly- its pretty standard for teen fiction and the plot is no better than something like Eragon, or a million of the Garth Nix novels from the early 2000s.
So I wouldn't say it was a fluke, Id just blame the timing. The writing was always Percy Jackson tier, and that just doesn't cut it today. Its when, half a decade later, that they made Korra, that you really realize that the shows plotlines are the things you see in teen drama books- only Korra didn't have the zany anime esque yaoi paddle like emotions that the original avatar had, imo.
The biggest problem I've noticed with western animation since Avatar is that none of it seems to be made for kids anymore.
I mean Steven Universe and the other lol-so-random type of shows feel like they are made for adult autists and fetishists and not actually children.
Thats basically the other half of the problem- in that these former kids writers just don't know how to write for adults, and their shows shouldn't even attempt to be written for adults. Most of them either just copy plots from old samurai films or insert random ideology (and their own bad takes on ideology) that they think makes the show adult-like, but is really just hackish- so you wind up ruining the show in that regard too.
I feel that if they had just kept it for kids, and kept the whimsical adventure feel and just placed things hundreds of years later, it would have worked much better.