When Aang emerged from the ice in Avatar: The Last Airbender, he was an infectious ball of energy who wanted to be a kid as much as he wanted to save the world. The charm and presence he had in the series was shared by the main characters who ed him - Katara, Sokka, Toph, and Zuko. It also, however, is present in a lot of the characters that are a bit more fleeting in the series.
There are a lot of side characters in Avatar: The Last Airbender by virtue of so much of the story featuring Team Avatar traveling through their world. And, it's a huge feat that the writers managed to make so many of those side characters compelling and appealing enough that the audience could have believed they were main characters in their own stories.
Jet
A contemporary of Katara and Sokka, Jet is a teenager the group crosses paths with who is leading his own band of kids against the Fire Nation soldiers. It's interesting for the audience to see that Aang and his group aren't the only kids feeling the pressure of the changing world. It's not just the adults that are finding themselves in the middle of conflict.
Jet is a viable love interest for Katara, though nothing ever happens between them, but he's also a hero in his own right. He sacrifices himself so that others can continue their battle against Ozai's control. The idea of Jet and his group of misfits is very Robin Hood, and would have made for an interesting story all on its own.
Suki
Suki, despite being such a big part of Team Avatar, only appears in about a dozen episodes of the series overall, which might be a surprise. Her place as a Kyoshi warrior, a non-bender, and Sokka's long-running love interest makes her a pretty fascinating character.
An entire series could have been written surrounding the Kyoshi warriors themselves, while the animated series only spotlights them briefly. The live-action Netflix series appears to see Suki for the potential main character she could be though, as she's been cast for the first season, as has Kyoshi.
Uncle Iroh
Is there any Avatar character as beloved as Uncle Iroh? The audience knows so much about him, but doesn't actually get to see how he changes from a battle-hardened soldier for the Fire Nation into a wise man who loves a good game and a cup of tea. It's clearly a drastic change based on the stories told about him, and it makes him a fascinating subject.
Iroh always knows more than he's letting on, always has a quip or a piece of advice ready, and is full of surprises. He would make for a great main character.
Ty Lee
Despite being part of an initially villainous trio, Ty Lee is one of the most fun characters in the entire series. She's incredibly knowledgable about pressure points and fighting styles, combining the acrobatics she learned in the circus with fight training to make her one of the most formidable fighters in the show.
Ty Lee, despite her bubbly persona and her fun approach to a fight, has led a sad life. She's played second-best to Azula and is seen as just another one of her sisters, giving her the need to make herself stand out. That's a story worth telling in detail.
The Pirate Captain
The audience never finds out his name, but the Pirate Captain is a larger-than-life scoundrel who has his own loyal iguana-parrot and crew. He and his crew steal rare items all over the world and sell them to wealthy people. When Katara crosses paths with them, she steals a waterbending scroll from him, earning his ire.
He only appears in a couple of episodes, but the idea of pirates making life difficult for benders and non-benders alike in the Avatar world is a fascinating one.
Princess Yue
Sokka's first girlfriend became the moon, but before that, she was the princess of the Northern Water Tribe. She was raised to lead her people and to follow tradition in a very different environment from Sokka and Katara.
The Northern Water Tribe is a very strict place, seemingly more developed than the Southern Water Tribe, and has a lot more benders. Princess Yue isn't a bender, but saving the world when the Moon Spirit is destroyed falls to her, and she accepts the challenge gracefully. Princess Yue is as much of a hero as anyone on Team Avatar, but her character is often dismissed as nothing more than Sokka's girlfriend, which is a shame. She could have been the star of a story depicting her community.
The Cabbage Merchant
His cabbage cart being destroyed is a running joke in the series, but there's no more popular recurring character than the man just trying to sell his cabbages.
It's incredibly easy to imagine the cabbage merchant as the star of his own sitcom where everything that can go wrong will go wrong, and it would give viewers a fantastic point of view on the main Avatar storyline too. He struggles to stay afloat and his family with his cabbage sales, but he never gives up.
Bumi
Bumi is the only person in the world of Avatar to have been Aang's contemporary as a child that the audience sees aged after Aang is thawed from the ice. He's incredibly eccentric, ruling a kingdom, but still having fun sliding down stone ageways. Bumi becomes more of an Avatar parental figure than a friend thanks to the new age difference between him and Aang.
Wouldn't it have been fascinating to see how he adjusted to the world while Aang was away? He's clearly maintained a kingdom and had not fallen to the Fire Nation despite the latter's reputation for violence and strength. Seeing Bumi grow and mature in the wake of Aang's absence could have been a great dramedy.
Azula
While Zuko and Uncle Iroh are originally in pursuit of Aang for Fire Lord Ozai, Azula becomes the real villain of the story. Her father might be the show's big bad, but it's Azula who is the most dangerous, willing to dispose of her own brother to bring more power to the Fire Nation.
Azula is such a fun character to watch. She's not as charming as Ty Lee, but she's just as compelling. The audience wants to see her climb to the position of Ozai's favorite, and her unraveling as the pressure mounts on unsuccessful missions is done so well with her in a ing role that she would jump off the screen as the main character.
The Ember Island Players
Filler episodes are a well-known part of anime. Avatar gets its version of filler with a show put on by the Ember Island Players. The actors play out Team Avatar's story, even giving a dramatic ending that hasn't happened yet. It's not just about killing time though, as each of the of the team react differently to what unfolds and it gives the audience insight into them.
More than that, however, the Ember Island Players are fantastically entertaining for the audience. They know things about the story so far that should be impossible. The writer of the show would have had to be right on the trail of the characters, interviewing people along the way to compile the version of the story they have. That journey is a must-see.