Fiora is broken but let's not let Kog get away with being game breaking because of it. They're both unfair garbage and both deserve to be knocked down to manageable levels. As for the others, I'll never be okay with Swain. I don't think he brings anything of value to this game. He's lazy, boring, and frustrating to play against. His kit has no interesting reactions (Zhonya's in ult is not an interesting reaction), and the game would be better off with less champions like him. That being said, he isn't "broken". Last I checked, Viktor was pretty good, but I haven't heard anyone complain about him. Same goes for Karthus, but as a Kha'Zix player almost exclusively, sometimes I never realize that a champion I easily counter is unbalanced. That might be the case with Karthus, so I can't comment as far as that goes.
On the subject of Fiora, she's in the same spot Yi and Xin Zhao are. The main difference is that Fiora and Yi can outplay, while Xin is mostly linear. Fiora is more versatile than Yi, which puts her a little bit higher up there when you get to the higher tiers of play. Now, why is it that these champions are suddenly a plague? Let's see what they all have in common. They're all auto-attack heavy champions with effective health boosts (in the form of heals, and for Yi, damage reduction) for one. That's pretty lame, but that alone shouldn't make them as broken as they are. So what else do they have in common? Take a look at the last few games you had a Fiora, Yi, or Xin in. Odds are, if they game went long enough, they have Dead Man's Plate and Sterak's Gage. Oddly enough, our old friend Kog'Maw, game breaking master, is also building Sterak's Gage, but we'll get back to that later.
So, what's up with these two items? Why are all of these champions building the same garbage every game with little to know diversion from their core path, even though they aren't that similar? If you guessed that these items are way too good, congratulations, you figured it out. Now, at the risk of a bunch of angry tank mains tearing me apart over it, I'm going to say Dead Man's is overpowered. It's commonly regarded as straight up superior to Sunfire Cape (no, that doesn't mean Sunfire needs a buff), and gives some stats that, to be totally honest, tank should have limited access to. However, as obnoxious as tanks can be with Dead Man's, it gets worse. Dead Man's, in combination with Sterak's, give auto-attack fighters access to multiple artificial damage boosters while giving high tier survivability. Master Yi and Fiora were not meant to be offtank Gods that could three hit other champions while taking do damage in return.
Back to trade offs. Champions like Malphite are toxic because they're borderline unkillable and do not suffer any real damage penalties in return. If Malphite rushes a 100% defensive item, h should be tickling for damage, end of story. So why Malphite? Because no champion should directly gain damage output for building defenses. None. It's just not okay. What I find confusing is that Riven got nerfed because she was able to build damage and gain effective health because of it. Okay, that's fine, I agree. But why on Earth is that okay for Diana? It isn't. On the inverse, why can Malphite gain damage for building tank items? It's silly. Now the problem isn't just Malphite. With this Juggernaut nonsense we were introduced to these ridiculous items that reward a player with damage... for building survivability. See what I'm saying? If Fiora wasn't a monster tank, she wouldn't be nearly as broken. Fiora should not be building tank. It wasn't intended and I don't care who has fun playing it, it hurts the game. The same goes for Kog'Maw. The only reason he's still going strong is because he saw so little play before this, and they're worried about overnerfing him too quickly. I understand that, every champion should be viable. But let's be real. It's better that a champion is garbage than gamebreaking. Yes, even if they're your main. When an ADC is building a juggernaut item, you need to take a step back and say "Alright, this isn't okay."
Riven was one thing, but let's talk about something I know a little more about, personally. Kha'Zix had his ultimate damage reduction removed. Why? Because people started building tank on him. Flat health (like Warmog's) is amazing with flat damage reduction. This situation is a little different, but the idea is the same. Kha'Zix wasn't supposed to be a tank and still deal damage. Riven wasn't supposed to be a tank and still deal damage. They were nerfed. Fiora wasn't supposed to be a tank and still do damage. Yi wasn't supposed to be a tank and still do damage. They're being let off the hook... for now.
Yeah, they're going to get nerfed, but not until after they're banned in literally every game so no one gets to play them anymore. Then people will stop defending them and start begging for their champions to be nerfed so they can actually play them again. In the mean time, we shouldn't really have to deal with free losses just because they somehow slipped through ban phase.
Long story short, any champion that can build tank and still do serious damage is broken. Dead Man's and Sterak's are toxic together (and sometimes alone).