I’ll be honest, when I was a kid between the ages of five and eight, when the original series aired, I didn’t really care who ended up with who. When I rewatched the series as a teenager, although I wasn’t against it, I found Aang and Katara ending up together at the end of the series being kind of strange, and now as an adult, even though I still am not against Aang and Katara being together, I still find their union at the end of the series pretty odd.
The reason I find it strange is because even though Aang had obvious feelings for Katara from the very beginning, Katara’s feelings towards Aang, were not all that clear. In season one it’s clear she didn’t really view him as a romantic partner, but by season two it seems like she goes from maybe having a crush on him to viewing him in a familial sense, and flip flops between that. She starts reciprocating in season 3, but it goes back to being confusing when she talks to Aang and she tells him she’s confused about her feelings.
Usually the criticisms I’ve seen about Aang and Katara being endgame are rooted in the fact that Aang has feelings for Katara from the very start and for the most part Katara doesn’t reciprocate until near the end.
Some suggestions I’ve seen is that Aang and Katara should’ve parted ways at the end of the series and reunited when they were older between the events of The Last Airbender and The Legend of Korra and got together then, or that Aang should’ve gotten over his feelings for Katara at the end of season 2, because of the whole issue of opening the last chakra by letting go of his attachments to master the Avatar State.
I simply would’ve developed Aang and Katara’s relationship similar to Sokka and Suki’s relationship where Sokka and Suki’s romantic feelings for each other develop pretty much at the same time. I would’ve started Aang and Katara off as friends and Aang wouldn’t have had a crush on Katara at the beginning, and the two of them would’ve started developing feelings for each other over the course of the series as they get to know each other and appreciate each other more.