Any popular show is inevitably going to attract shippers, and Avatar: The Last Airbender is no exception. One of the most hotly contested fan-imagined relationships from the series was between the Waterbender Katara and the Firebender Zuko. Nicknamed "Zutara," fans were torn over whether the creators intended to imply any such relationship, if the two ending up together was ever a realistic possibility, if it was purely imagined or if the very idea was problematic given their imperialistic dynamic.
That contention doesn't solely exist in the fandom alone. Sources in the canon itself reveal conflict over whether Katara and Zuko could ever have had a romance, and the disagreement may have been just as heated behind-the-scenes in the writer's room as it was for fans.
The show itself was not without moments between the characters that could fuel such theories. While Zuko chased down the Gaang in pursuit of the Avatar for much of the first season, as his redemption arc ramped up in Book Two and the character grew even more sympathetic he shared a seemingly tender moment in the Crystal Catacombs beneath Ba Sing Se with Katara. After connecting on their shared trauma, Katara brings up the possibility of healing Zuko's scar with magic water from the spirit oasis. Ever the conflicted character, Zuko returning to his roots and trying to bring down the Avatar thereafter hurt Katara that much more.
But his redemption arc was not over, and when Zuko returned to the Gaang later to join them, Katara had clear issues with trusting him. After helping Katara on a mission to avenge her mother, the two become much closer, and a brief moment in the theater with the Ember Island Players had Zuko edge Aang out for a seat next to Katara. If it weren't for Katara and Aang ending up together, the groundwork certainly seemed there for the writers to go in the direction of Katara and Zuko being a couple. Whether that was being considered depends on who you ask.
An event known as Avatar Extras ran a marathon of Avatar episodes on Nicktoons Network with pop-up commentary from writers Joshua Hamilton and Katie Mattila. Sometimes the bubbles were just jokes, but others connected information on-screen to information revealed elsewhere in the series or provided extra information about backstory, canon or the creative process. That last bit proves particularly interesting because an early bubble says that Katara and Zuko were originally meant to end up together.
The bubble pops up in the episode "The Avatar Returns," the second episode of the series when Zuko is still firmly an antagonist. At that early stage, there was little groundwork built for romantic relationships between any of the cast, but such detail could show where the writers were thinking of taking things down the road. If this were truly the case, then Katara and Aang ending up together would be a deviation from the original plan.
However, there seems to be some contention over the matter, as creators Michael DiMartino and Bryan Konietzko have said elsewhere, such as in the book adaptation Sozin's Comet: The Final Battle, that they never intended for Zuko and Katara to end up together. Given the creators' heavier involvement in the series from start to finish, that would certainly seem to be the stance that takes greater authority, and Avatar Extras has been known to contradict the canon plenty of times in other places.
What is perhaps most interesting is that it just goes to show how such conflicting ideas can be as present among writers as they are among fans. The dramatic tension of who Katara would end up with helped amplify the audience engagement in the story, and by the creative team bringing different ideas to the table, they were able to craft a believable enough tension that fans still fight over which choice would have been right to this day.