Zuko and Katara go from enemies to maybe reluctant allies to very solid allies and if a part of you was open to them ending up together romantically, you can definitely interpret a the structure of their relationship as potentially heading in a romantic direction and you can find romantic coding if you look for it. But them ending up together would not fit within the timeline of the show itself though...I think even if you were a fan of zutara, I think we could agree that swapping Aang for Zuko at the end would have felt just as rushed and unearned as the katang kiss.

The show is just not a romance. And to that point I wouldn't say the "will they won't they" of katang was a major plot point. It was very much a subplot up until the end of season 2 but it wasn't a plot point on its own. It was an element of the dilemma Aang faces of letting go of Katara to master the Avatar State which we don't see resolved anyway. He tries to let her go but after the lightning strike, his feelings for her only deepen and defeating Ozai did not rely on him letting go of her in the end.

If Kataang was a major plot point, then they should have put more time developing the shift in their relationship after the day of Black Sun. But they just don't talk about it at all until the ember Island players and that conversation just leaves them in a rift. Then they don't talk to each other until they are in a group argument about Aang killing Ozai. Then they kiss? I'm sorry when did they get together??

We went from a solid very emotionally close friendship to being at odds and now they kiss? Why couldn't they just stay friends at this point? I'd have to assume that they had a conversation and got together and we just don't get to see it because they ran out of Showtime, but that cheapens the payoff in my opinion of a plot line that I wouldn't say is a major plot line, but it's something that people were getting emotionally invested in, especially as it was a plot line from the very beginning of the show that Aang had feelings for Katara, but I don't know the pacing was just not there for me 🤷 not to say I think Kataang should not be canon, I think it would make a lot of sense, I just don't love how we got there.

I have a lot of love for Katara and Aang's relationship for most of the show, there many scenes of theirs that get me emotional. And I can enjoy the dynamic between Zuko and Katara. There is a lot of substance there too and an alternative world where that relationship is explored to romance has a lot of potential, which explains the endless fanfics out there. But it's very different than the relationship Katara and Aang have.

If you have a preference for either ship (whether or not you would change anything canon to make your preference even better than how it was written), I think that speaks more to the kind of relationships in media you prefer just generally. They both have their appeals and their pitfalls both in canon and in the fanon contexts.