(I’m not going to quote out the whole original post because it’d make this twice as long, but, needless to say, this is a response and one would be better off reading the original post first)
While I do think that considering how one would feel about a Rule 63 version of the show can be really interesting, I think such swaps are far too complicated to say, “because the Rule 63 version is bad, the canon version is also bad.”
I’d like to start off by pointing out that gender-swaps affect things in two major ways – first, by changing the dynamics of power, and second, by replacing stereotype with non-stereotype and vice-versa.
Cases that fall into the second category only say something bad about the original show if the switched gender demonstrates a stereotypical element in canon that might have been otherwise overlooked (think male characters drawn in female poses to show the absurdity of those poses). In the case of Legend of Korra, though, there aren’t very many cases like that. Instead, we get non-stereotypes turned into stereotypes when the genders are switched. F!Yakone might look like a horribly stereotypical evil mother, but being defined by parenting skills is something that doesn’t happen to male characters nearly as often. In this case, it’s easy to see how switching genders would make a show look more regressive than its canon version actually is.
(This wasn’t something you mentioned, but F!Tenzin would have the same sort of issues. Placing a female character in a position where she’s compelled by duty and necessity to bear airbender children would be far more questionable than doing the same to a male character. Amon’s choice of victims would be another case of this – in the Rule 63 version, all female victims would be throwaway characters meant to raise the stakes for m!Korra while the male victims would be limited to m!Korra and m!Lin, whose de-bending is an important part of their own story. It’s easy to imagine this being an intentional decision to avoid Women In Refrigerators, actually.)
As for the category of changes in the dynamics of power, even that’s a bit more complicated than “reversing the power dynamics shows how problematic things really are.”
M!Korra beating up a female Protester is a pretty simple example of this. Male violence against female victims is incredibly charged in a way that often renders every other dynamic meaningless. M!Korra beating up a male Protester looks a whole lot like canon Korra beating up a male Protester. Canon Korra beating up a female Protester looks a bit worse because of Korra’s misgivings towards femininity, but all three of those still look very different from m!Korra beating up a female Protester, because society implies that all men who fail to follow the “Never Hit A Girl” rule do so because they hate women (even, in many cases, if the man in question would be perfectly justified in defending himself through violence, leading to restrictions on female enemies and Designated Girl Fights). Korra beating up the Protester is wrong either way (and it’s used by the show as a way to show how out of touch she is with the society she’s in and how natural violence against others is for her); switching genders adds an extra reason for Korra to be wrong, but it’s a rather specious one at that because it’s based more on chivalry than anything else.
The nature of f!Tahno is an example of something a bit more complicated. I think the societally-expected power dynamic between men and women might have caused a bit of confusion as far as her characterization is concerned. Here’s the problem. F!Tahno, as you characterized her, is overtly sexual and little more. Canon Tahno, however, is legitimately predatory. This difference in implication revolves almost entirely around Tahno’s offer of “private lessons” and what it says when it’s male-on-female versus female-on-male. Canon Tahno’s offer is, more than anything, a power play – by offering to show her “how a real pro bends” in front of his posse of teammates and hangers-on, he’s publicly reducing Korra to something he can dominate sexually while ensuring that she’ll be punished if she retaliates. It’s blatant sexual harassment. In f!Tahno’s mouth, however, the “private lessons” line doesn’t seem nearly so reductive, even if she’s saying the exact same thing. If they hooked up, f!Tahno would still be seen as giving m!Korra her body instead of taking anything from him according to society (even if she treated him as an inexperienced student the whole time), so all she’d do is prove herself to be “easy;” canon Tahno is instead assumed to be capable of casting canon Korra as a sex object to everyone nearby merely by putting the option of sex (and the implication of humiliating sex, at that) on the table.
Unless the harassment aspect was made far more prominent, then, if m!Korra likes putting f!Tahno in her place, his reason for feeling that way would be understood as, “I have no respect for women who tempt me with sex and I enjoy punishing women who are sexually dominant” (which is… less than sympathetic, to put it mildly). Canon Korra’s reason for hating canon Tahno, on the other hand, is all about retaking power from someone who harassed her – he tried to make her look and feel powerless in front of an audience, so she takes great joy in enforcing her own power over him in the arena. Amon’s actions against Tahno are similarly affected by the shift in Tahno’s implied motivations. Canon Amon punishes canon Tahno through an act of supreme dominance for his attempts to abuse his power over others; f!Amon (or, worse, canon Amon!) would appear to punish f!Tahno through an act of supreme dominance for cheating in a game and offering sex too easily, which creates all sorts of nasty implications that have nothing whatsoever to do with the original show.
And that brings us to what is quite possibly the biggest issue of all: Korra’s character arc is focused almost entirely around her highly disordered drive for dominance, and fandom’s interpretation of that is affected in all sorts of weird ways by her female body (but would, of course, be affected in different but equally weird ways by m!Korra’s male body).
The way agency is attributed is quite possibly the weirdest among these, of course, which plays a huge role in the way the love quadrangle is interpreted. Mako is assumed to have more agency than he does and Korra is assumed to have less because he’s male and she’s female and that’s how those things usually go. Mako’s role in the romance plot is overwhelmingly passive, apart from the one time he gave Korra an ultimatum to try to get her to stop persecuting his girlfriend’s father – both Korra and Asami make advances on their own and his biggest failing is indecisiveness about which offer to accept. His agency was seriously compromised when Korra kissed him, both because she essentially bypassed his rational mind completely by catching him by surprise and because the whole situation is really awkward due to how much he needs her in the immediate future and how much he owes her already. He’s perceived, however, as being manipulative and domineering, partially due to an over-focus on the scenes in which he actually is active and partially because, as a guy, his inability to choose between two women is immediately assumed to be a badly-justified attempt to retain easy access to both girls’ bodies and beds rather than a mixture of romantic indecisiveness, difficulty in navigating relationships, and an attempt to avoid hurting the feelings of two people to whom he feels highly indebted that backfired horribly.
Korra, on the other hand, is assumed to have less agency than she really does, which is a pretty big problem given that her power and control issues influence her behavior pretty heavily, including her part of the romance plot. The show itself is perfectly willing to accept that her behavior is problematic, and demonstrates that through smug/sleazy facial expressions and a clear and consistent link between her romantic insecurities and power-plays (both against Mako himself and against uninvolved Probending opponents). That the audience misses the show’s disapproval for Korra’s tendency towards power-play seems to have more to do with the reversal of gender expectations than anything else – the show isn’t seen as calling Korra bad by the audience because the audience sees Korra as childishly impulsive rather than dangerously inclined to abuse the power she has over others to feel better about herself, and she isn’t seen as doing anything dangerous largely because she’s a girl and therefore is believed to be operating from a position of weakness and ineffectual frustration even when she’s operating from a position of overwhelming power.
So, yeah, Korra has consent issues, but Korra’s consent issues are part of a broader arc of problematic behavior that begins with defiance of authority and consistent attempts to deal with insecurity through violence against others and grows so extreme that she takes grim satisfaction in mocking a guy scrambling away from her in terror for his helplessness before trying to burn him alive. The kiss, as much as it’s another manifestation of Korra’s need to retake control in whatever way she can, is something she recognizes as an issue and learns from. It’s also something that the show blames Mako for far less than the fandom does – Asami and Bolin blamed Mako initially, but neither of them actually realized that he didn’t have much of a choice in the matter at that point (and he never actually pointed out that out to either of them on-screen), and in the end neither expected him to apologize for being kissed. He and Bolin, after all, basically just throw their hands up and say, “Girls!” and Mako’s final apology to Asami is an apology that things got messed up rather than an apology for any particular action.
(This might be why f!Mako isn’t entirely unappealing – she’d be treated in the ways one might expect our society to treat her by the characters, but the show itself would be sympathetic to her confusion and frustration, even if it would still be sympathetic to Asami and Bolin’s reactions to what they thought they saw too)
As for the other two issues revealed by the genderswap, I think they’re both rather similar to canon in a lot of ways. Sure, m!Korra thinking m!Asami was a prissy little bishie until m!Asami took him racing would seem like a swipe at feminine guys, but it’s balanced out by the fact that m!Korra has no problem with m!Pema being a house-husband, wore a really fancy suit at f!Tarrlok’s gala, and tried on some of m!Asami’s cologne (and put way too much on!) once he realized m!Asami was actually pretty cool.
F!Amon’s lack of motive besides her upbringing, on the other hand, doesn’t seem any more unfortunate than canon Amon’s does, really. If you’re disappointed by one, you’d probably be disappointed by the other, and if you’re satisfied by one, you’d probably be satisfied by the other. F!Amon would quite possibly come off very differently in some other ways, though – canon Amon’s persona is very much one of dominant masculinity, from his manpain backstory (“a firebender killed my family”) to his lack of open emotionality to his ability to violate and terrify through touch, which might leave f!Amon seeming less immediately imposing (if maybe a bit more Freudian when set against m!Korra and m!Lin).
And so, on the whole, I don’t think I can agree with your assessment that “the really problematic stuff is already there.” The Legend of Korra is, to a large degree, a show about power, and the language of power is a veritable minefield of gender stereotypes and sexualization in which carefully choosing characters’ genders is the only way to keep any sort of focus on the dynamics of non-sexual power. It’s not unproblematic, by any means, but “Rule 63 Tahno promotes hatred towards women who sleep around” and “Rule 63 Korra looks like a misogynist for picking on Rule 63 women so often” aren’t really among the show’s actual problems.
These posts are both really interesting, particularly as a fan of genderswap, largely because of the intellectual exercise here - both the ‘how does the same thing register to audiences with changed genders’ and ‘how would it actually change things’ angles, though I tend to prefer narrow-target genderswaps; a friend of mine and I were talking a few months ago about f!Mako and why she’s more appealing for us than canon!Mako and how she’d work and so on (e.g., f!Mako + f!Bolin is a different dynamic than f!Mako + canon!Bolin). I do think seeing the issues Maka wrangles with could be very interesting, done right.