Anon asked: I need some zucest headcanons with evil!Zuko!
Oooooohhhh, I like this.
If Smol!Zuko was already evil
He and Azula would have stayed friends and I can see them having little schemes that would 50% evil, 50% stupid, and 100% insane (sort of like cartoon villains, say Shego and Drakken from Kim Possible). They'd be on more equal footing - Zuko would not be a prodigy, but his sadistic tendencies, impatience, entitlement, and immense pride would have made him the perfect prince in Ozai's eyes. Regular!Ursa would be appalled by what both her babies are turning into, while Evil!Ursa would just be proud. When it came to the whole "Sacrifice your firstborn" fiasco, Ozai would either use it as a way to get rid of Ursa so she will stop trying to hold back their kids, or (if they're also an evil couple) they'd just plan Azulon's murder together, Ursa would not get banished, and then just have Ozai as Fire Lord with two powerful heirs.
Since there would be nobody with morals left in the family and Zuko's kind nature would never develop, he would not be banished. And since they'd have no morals and would consider themselves superior to others (and therefore allowed to do whatever they wanted) Zuko and Azula would probably get together the second puberty hits them. Ozai (and evil!Ursa) would not have a problem with that since it makes it almost guaranteed that they'll have strong heirs.
Since Zuko would never become the blue spirit and never rescue Aang after he was captured by Zhao, the biggest threat to their nation would already be gone. Either they'll take over Ba Sing Se somehow, or they'll just burn it to the ground along with the rest of the earth kingdom when the comet comes.
I can see Zuko and Azula having sort of "Gomez and Morticia" moments, acting like all the death, misery and horror surrounding them is the most beautiful thing in the world and having date nights among the ashes of their enemies. They'd later kill The Sun Warriors and have dragons for themselves because yes.
If Zuko became evil after being banished
On top of being bitter and resentful towards the entire world, and then having Azula lie to his face about Ozai wanting him to come home, Zuko would be furious and want revenge against his family. He'd probably want to kill his father and be a bit on the fence on whether he wants to do the same to Azula, or force her to marry him as a "punishment" for having looked down on him for so long (and totally not because he loves her or anything...)
As they're chasing the Avatar, he'd try (dressed as the blue spirit) to kidnap her, but she'd escape because she's Azula. Iroh would take him to Ba Sing Se (either to try make him forget about his revenge OR, if Iroh is also evil, to buy himself some time to work on a plan), which would lead to the whole thing with Azula genuinely offering to bring him home (if Iroh is not evil, he will not join them and be imprisoned like in the show - might get executed later).
Once they're home, Zuko and Azula would probably be sort of "enemies with benefits", and after they win the war, Zuko would convince Ozai to allow him to marry her (regardless of whether she wants to or not) to assure they'll have strong heirs. Be it either by wearing her out, or not giving her a choice, or convincing her using the same arguments he used on Ozai, or having her gladly accepting his proposal, the fire siblings would get married (then kill The Sun Warriors, get dragons, have babies, etc)
If she already loves him by the time Zuko finds a perfect opportunity to murder Ozai, Azula will help him (probably with poison). If she doesn't, then Zuko will just disguise himself as the blue spirit, kill him, and disappear in the night, and Azula would only find out Zuko was behind it years later, after he is sure she's loyal solely to him and willingly tells her the truth.