Look for fanfiction that is rated highly for both story and characterization. Since you're watching the series, you'll be able to notice if the fanfiction stays true to the characters' personalities. For example, if a Zuko in fanfiction acts completely out of character compared to the show, it might not be a great one. Try to find ones that feel like a natural extension of the series.
One great 'ATLA' fanfiction is 'The Avatar's New Adventure' which delves deeper into Aang's training after the series ends. It's full of interesting new bending techniques.
One common theme is power exchange. For example, when characters from Atla enter another universe, they might learn new powers or ways of fighting. Another theme is culture clash. If they cross over into a very different world, like a modern - day Earth in some crossovers, the clash between their traditional Atla culture and the new culture can create interesting stories.
One popular 'atla oc fanfiction' story could be about an OC who is a powerful waterbender from a hidden tribe. They journey to the main story locations and interact with the canon characters, adding new storylines and character dynamics.
Well, Wattpad also has a good amount of series fanfiction. It's a platform that not only hosts fanfiction but also original works. However, the quality can vary a lot. But it's easy to find works related to popular series. You can search for your favorite series name and then browse through the fan - made stories.
A really cool 'atla crossover fanfiction' is the one that crosses it with the 'Star Wars' universe. The concept of the Force and the different bending abilities have some similarities. The Jedi and the benders could learn from each other. It often has great storylines where the characters from both universes interact and face common enemies. There are also some really creative fan - made artworks related to these crossovers on DeviantArt.