Thing it would be neat to see explored more, both in canon fiction and fanworks:
The distinctions between a reformed villain and a redeemed one.
Redemption is hard; often impossible, even, depending on what's been done. Because redemption is about debt, and trying to pay it off.
Reformed though? That just involves switching sides, and there is *always* more sides than just "good" and "evil". Nobody has to forgive you or trust you (whether they do or not or *should* or not is a separate case-by-case discussion and tangential at best for this discussion); you don't even have to do anything active depending on how you want to define the term, you just have to stop digging the hole you're in. You just have to walk away and make different choices from now on. You just have to stop being an antagonist, mostly. The bar is a lot lower for reformed, without compromising a potential character or plot arc.
An ex-villain does not have to be redeemed to pull a compelling and believable face turn. Not all ex-villains should even want redemption, not to speak of getting it, and that's not a flaw in writing. Zuko (and Uncle Iroh, hottake) are considered redeemed by the end of that show, Catra is reformed *not* redeemed though is striving for something like it eventually at the end of hers, Dragonball ex-villains on main are merely reformed with zero concern about fluffy things like redemption or morality at all.
(If you think Catra -- as the common example -- is supposed to be redeemed by the end of the show, I'd get why you'd not like it/her, but my whole point here is that she was never supposed to have been and judging the show by that standard is narrow minded and missing the point; that's a failure in the lens you're applying not the story because redemption is not the only plausible story-endgoal for a character like that. But also that I don't blame you *because* these sorts of nuances are under explored; people certainly call it a redemption arc so it makes sense people would judge it as such. Everything is all binaries: a character is hero=protagonist/protagonist=good/correct or villain=antagonist/antagonist=evil/malicious/wrong, ex-villains are either entirely redeemed(good writing) or entirely unpunished(bad writing), other such nonsense etc. And it's complicated because redemption does seem to be a long-term goal of that character, but she only starts that journey at the end credits with the hazy implication of "but that's another story", so it's left on a cliffhanger unresolved. But that's intentional not sloppy; it's a feature not a bug. It's fine if you find that lack of resolution frustrating as a personal preference, but that's different than objectively flawed.)
Which reminds me to not get trapped in a new false binary! I can make a compelling argument that "poster boy for good redemption arcs" Zuko is actually a rehabilitation arc and Iroh's the one with the perfect (if closeted) redemption going on, because Zuko tries to do the right thing and gets punished for it and only does communal harm under manipulation and/or active threat; Zuko doesn't need to be corrected and is responsible for very little so there's nothing to redeem, he needs help instead and he gets it. Redemption is about paying a debt/healing a wound you left in your community; rehabilitation asserts a wound *in you* which can be fixed but also implies a lack of responsibility/understanding/autonomy and communal responsibility for helping fix it; it's the same principle as the insanity defense. Someone who doesn't know they're doing the wrong thing or someone compelled into doing the wrong thing by someone with life-threatening power over them is not the same as someone who is willfully choosing to do the wrong thing; soldiers on both sides of a war are equally sure they're the good guys after all. Redemption is about your responsibility to the collective; rehabilitation is about collective responsibility to you. (Most situations need some combination of both to satisfyingly flip an antagonist, there are few true villains *or* victims in a story, even in children's media.)
(Firelord Zuko as representative of the Fire Nation and its collective need for redemption shouldn't be conflated with Prince Zuko as an individual's struggle to separate himself from his abusive family; Firelord Zuko has all the power, resources, and authority to deserve that accountability; Prince Zuko has been socially isolated, his presumed authority made a mockery of, and is running scared, functionally homeless, disowned, and *desperate* and still is shockingly fixated on as much harm mitigation as he can manage, and doesn't owe shit to shit for 95% of what goes on around him until Ba Sing Se, fite me. Great character but bad example of a redemption arc imo.)
Adora's faceturn likewise is an ex-villain rehabilitation arc, which is why she's held to lower standards than Catra but/and also why Hordak seems to get off lightly in comparison to Catra or Shadoweaver; Catra and Shadoweaver both *know* they're hurting people and are doing it on purpose but Hordak is mostly just trying to be a good boy to the definition of good as he understands it. "They should know better" is an excuse to vilify people in shitty situations, for the "heroes" to be lazy and punitive, because often? They really don't; mostly people only know what they've been taught, and nobody is entirely immune to abusive conditioning. Ex-cult *leaders* might need redemption but ex-cult *members* often need to be saved more than they need to atone even when they've done some terrible things, especially if they're born in it; child soldiers are not culpable for the people they're aimed at and so redemption is the *entirely* wrong metric to grade them from.
Keeping the R words going, retirement is also 100% a valid ex-villain option, no remorse or even proper reformation/reintegration necessary. Ex-villain doesn't necessarily mean hero, it can as easily mean civilian or even evil-but-nonaligned. Orochimaru in Boruto is my absolute favourite of this type right now; 100% unchanged and unrepentant, nobody pretending they are but they *do* have a truce and nobody wants to start that fight so off to the parent-teacher conference they go. (Snake mad-scientist: monarch of the Hidden Leaf PTA. Practical applications of chaotic evil for pro-social purposes; unfortunate cost: somehow personifying The Entire Concept of Mom Jeans while still technically in old school Japanese attire? Shoutouts to the animators for absolutely nailing "I am SportsMom VanDriver, fear my petty evil as much as my largescale" as an aesthetic *and* aura.) And there's *tonnes* of reasons why a villain might chose to just wash their hands of the whole central plot conflict without coming to any conclusion beyond "my boss sucks" or "a pox on both your houses, I'm outie" or "this costs more than it's worth to fight". This is most of the Dragonball ex-villains (as I understand it anyways).
Do you actually want the story to redeem the ex-villain or do you just want them kicked while they're down? Consequences!=harm. Revenge is not justice, and punishment for its own sake is just torture not reparation.
Is redemption a goal that even makes sense for this character?
There is no one right outcome; don't force it on characters it wouldn't make sense to. Even being-a-villain isn't a binary toggled state, and there's a million more ways to not-be-a-villain-anymore and most of them don't even need contrition not to speak of atonement; they don't even inherently require you to be want to be good.
That's not bad writing that's just life.
