Okay more post-canon zk headcanons since we've been seeing alot of anti takes lately:
Katara has like zero spice tolerance and thinks Zuko has something clinically wrong with him because she'll be emitting smoke out if her ears, tears streaming down her face and he'll be adding MORE chilli flakes. He once told her that it wasn't his fault she had the refined pallette of a snail and it almost ended their marriage.
Conversely, Zuko has a ridiculously high salt tolerance and SWT food actually really agrees with him. Katara once tried adding extra to an already very salty meal and he didn't even notice. In fact she was the first one to reach for a drink. This also nearly ended their marriage.
They're both very reluctant to have a nanny for their children. With so many people still out to get them, trusting somebody with something so important is a task. Enter: A huge, terrifying 70-something SWT veteran, covered in scars. He is the sweetest, kindest, most ruthless nanny they could possibly find and the kids have remarkably assasin-free lives after he joins the family.
Katara absolutely loves a spa day. Zuko has a sauna built into the embassy just for her and she regularly gets herself one treatment or another. She and the palace aesthetician are best friends.
In fact Katara actually gets on very well with everyone on the palace staff. Zuko does as well- he's put ALOT of effort into it- but there's still a lingering formality around him. Katara has a way about her that connects with people, particularly the down-trodden and invisible. Frankly if she ever wanted to revolt against the Firelord it wouldn't even be difficult.
Being popular with the servants also means that Katara happens to know everything about everyone who comes into the palace- whether that's nobles, dignitaries, ambassadors etc. Servants know everything.
At one point before she and Zuko are together she ends up leading a revolutionary strike among the palace kitchen workers. Zuko is both impressed and exasperated because she could have just asked. ("I would have fixed the kitchens up anyway!" "It's the PRINCIPAL!!").
Similar to the lightning having chronic consequences, bloodbending has serious side-effects when it isn't done carefully or healed properly. After the incident with Hama, Sokka and Aang both experience seizures. Katara was able to catch them early on and apply some healing that led to them mostly going away quite quickly. However, she forgot to give herself the same treatment so years later she starts experiencing absent seizures, which eventually start knocking her unconscious. She is able to keep them a secret, until she experiences a very bad one in public. Zuko unleashes his own personal NWT physician on her (Katara deeply regrets introducing them), and insists she gets the help she needs. Turns out he has the bedside manner of a mountain lion and runs her recovery process like it's the navy.
U cannot tell me that the marriage proposal went smoothly. If u ask me it went something like this: Zuko pops the question a little like it's a grenade (definitely did not plan to, it just kinda exploded out of him), and there isn't even a moment for Katara to answer. He absolutely takes this as a big fat No (which it isn't). She just needs a minute. They get there eventually, probably mid-battle pirates of the carribean style.
