In Avatar: The Last Airbender, Avatar Aang returned to save the world from Fire Lord Ozai, but that wouldn't have been necessary if he had learned a key lesson from previous Avatars. In the world of The Last Airbender, the Avatar is the one person capable of mastering all four elements. However, their powers extend further. Avatars are capable of being immensely spiritual and are thus charged with acting as the bridge between humans and spirits. They can also interact with the spirits of previous Avatars, their past lives, and if Aang had a stronger connection to his previous reincarnations, he could've saved many more lives.
The success of The Last Airbender has allowed the franchise to expand since the original series, with a sequel series The Legend of Korra releasing several years later. Its resurgence in popularity in recent years has also resulted in Netflix's creating a live-action adaptation of The Last Airbender, while three animated movies set in the same universe have also been announced. Beyond film and television, The Last Airbender also has spinoff comics and novels, with the most recent being F.C. Yee's The Dawn of Yangchen. Yangchen is the last Avatar hailing from the Air Nation before Aang, and Yee's novel describes a tragic trend among Avatars that Aang falls victim to.
Avatar Yangchen is the Avatar that has the deepest connection with her past lives, with her connection being called a "gift" in comparison to other Avatars, such as Aang and especially Korra. After communicating with dozens of Avatars from years gone by, Yangchen noted one worrying pattern that "their lives are full of regret" for chances to make the world a better place, and that not a single Avatar "wishes they waited longer to solve a problem." Unfortunately for Aang, in The Last Airbender, he waits longer than any other Avatar as he encases himself in an iceberg for 100 years, allowing the Fire Nation to reign terror over the rest of the nations.
How Avatar Aang Could Have Stopped The Fire Nation 100 Years Earlier
Aang's guilt is evident throughout Avatar: The Last Airbender. There are several occasions he states it explicitly, such as in season 1, episode 12, "The Storm" when he's chastised by a fisherman for going missing for a century. Aang runs away to a cave and tells Katara "the fisherman was right. I did turn my back on the world." While Aang's absence is not intentional, only making the iceberg to save himself and Appa from a storm, his guilt for doing so is obviously multiplied because the Fire Nation wiped out every Air Nomad, including Aang's tutor Monk Gyatso.
There is a hypothetical scenario in which Aang could have saved the Air Nomads and other victims of the Fire Nations' war. If Aang had a connection to his past selves to the same degree that Yangchen did then he would've been able to feel the regret that past Avatars felt. Upon feeling this regret, Aang likely would have fought the Fire Nation at the beginning of the war when Fire Lord Sozin ruled the nation. If any Avatar after Yangchen could have tapped into this spiritual potential, it's Aang - since he is the Air Nation Avatar that proceeded her and was taught the Air Nations' ethos about spirituality and altruism. Aang only needed to connect with Avatar Yangchen specifically, who could've advised him after all she had seen in the past.
Nonetheless, if Aang had fought the Fire Nation 100 years earlier, he may have done so without mastering the four elements. Not only would this completely change the events of Avatar: The Last Airbender, but it's also possible that this would've ensured his defeat due to his inexperience and the fact that he would've been without Toph, Katara, and Sokka. This would have led to the Fire Nation ruling the world of The Last Airbender and potentially ending the Avatar cycle.