Avatar: The Last Airbender is one of the most impressive and memorable animated shows of the 21st century, but like every show that's ever been released, not every great idea appeared fully formed. In fact, many of the major characters of the show went through some drastic changes during development.
Here are all the ways Avatar: The Last Airbender was radically different over the course of development and production of the series.
SOKKA
Sokka is the planner for Team Avatar, using his weapons skills, inventive nature and strategic expertise to make up for his lack of bending powers. According to Avatar: The Last Airbender - The Art of the Animated Series, Sokka was initially conceived as being 13-years-old, and the same age as Katara. This would give the pair something of a sibling rivalry during their journey instead of making Sokka more protective of his younger sister. Eventually, however, Sokka was aged up to be 16. He was also originally created as a more serious figure, but the performance by voice actor Jack DeSena gave Sokka a bigger sense of personality and comedic relief that the creators enjoyed.
AZULA
Daughter of Fire Lord Ozai and younger sister of Zuko, Azula is considered a Firebending prodigy by the majority of the world. She also proves to be a vindictive and cruel young woman, lashing out at those around her when she doesn't get her way. However, she almost ended up forcibly tied to another person. During an interview with AvatarSpirit.net, Michael Dante DiMartino and Bryan Konietzko revealed that, in their heads, the Fire Nation still practiced the idea of arranged marriages -- and one idea for the third season was to have Azula forced into one such engagement. The idea was ultimately dropped, although the idea of Azula trying to experiment with romance does occur during a party in the third-season episode, "The Beach."
IROH
Iroh is one of the most likable characters in the series. The gentle-hearted Firebending Master and Zuko's uncle, Iroh was the one calming element in the life of the wayward prince. He quickly established himself as a lighthearted and empathetic man -- with more than enough power to bring down anyone stupid enough to threaten him or an innocent. However, the character was initially intended to be a more serious figure, similar to Sokka. Remnants of this characterization can be seen in early episodes during his training with Zuko, but his more amicable side became more prevalent as the series continued. There was even consideration given to delving deeper into the character's backstory, which was ultimately left more of a mystery.
TOPH
Introduced during the second season of the series, Toph quickly became a pivotal player in the show. Although she was blind, the brash and talented Earthbender used her disability to her advantage, becoming such a precise fighter that she even created the style of metalbending. Toph went through a number of changes during development, including being first imagined as a trash-talking 16-year-old boy who could serve as a foil to Sokka and a romantic rival to Aang for Katara's attention.
However, when series head writer Aaron Ehasz proposed the idea of a little girl being capable enough to bring down an entire squad of muscular Earthbenders on her own, the seeds of Toph's eventual form was introduced. Co-creator Bryan Konietzko even initially resisted this idea but grew to love Toph. The original design for Toph still made its way into the franchise though: the design of the Earthbender in the opening credits is modeled after Toph's original look; the actor playing Toph during the play featured in "The Fire Ember Island Players" resembles that original look, and the concept ended up having an impact on the look of Bolin in the sequel series, The Legend of Korra.
A FOURTH SEASON
Although the series had initially been constructed as a three-season arc, there was, at least at one point, consideration given to creating a fourth season of Avatar: The Last Airbender. Ehasz revealed some of the details of this proposed season to Den of Geek: The season would have largely focused on the attempts by Zuko to rehabilitate Azula and help her find peace while the wider world would have adjusted to the fall of the Fire Nation and what that means going forward for the other three nations.
The season would have also focused on Aang having to contend with the darkness he absorbed from Ozai -- potentially setting up the kind of internal conflict and self-loathing; a theme that would become prevalent in future animated series finales like Steven Universe: Future. The fourth season never came to pass, however, as the creators of the show instead shifted their attention to helping produce the 2010 live-action The Last Airbender film.