Champion kits I think they are problematic.

First of all, remember that this is my opinion, I don't think this is The Absolute Truth told me by The Flying Spaghetti Monster at my last happy night, so let's be civil. I'll skip mentioning champions that have small problems (plz remove Ahri's speed boost from her Q), and I will mention only champion that I think they have a huge problematic kit: either they are "op" or "useless".

Ashe: Her kit is just a remain of an older game. Her passive is ridiculous: it encourages her to not attack, but she's a markswoman based on basic attacks. Her Q (permaslow) is a bad concept for the current state of the game: it has to be weak or she will become a monster-carry which can trap you forever as fast as she hits a basic attack. And let's not forget that slow is in a bad spot overall right now since most mobile champions can plainly ignore it. Her kit overall lacks diversity to compete with newer, more-flexible kits from champions like Jinx or even Varus. She needs a moderate rework that maintains her core identity as a slow-heavy champion, but giving her a more flexible and interactive kit

Darius: What can I say about Darius? Probably the worst designed and most binary champion in the whole game. His play pattern is exactly the same every game, not depending on anything else. Just E > Q > W to harass until you can R to execute an enemy. Once and another. If it works, you win, if it doesn't, you lose: he insta-win or insta-lose a match-up at champion selection. He needs a full rework to make him a high, sustained damage with more decision-making and a far more interactive kit (and please, that doesn't mean more mobility).

Fiora: She's a extreme case of lane super-bully but useless at teamfights. Her identity is a mess: she's an assassin, a weaker version of Master Yi, even if you try to play her like a fighter: her kit just don't work to be played as a fighter. She needs a full rework, with different abilities, her high-assassination potential must be reduced if you want to make her viable as a fighter (she's the Duelist, not the Assassin, plz). I know they had planned a rework long time ago but seems like they forgot it.

Gangplank: I don't know what to say about this champion. I have no idea what are you supposed to do with him. I mean, I know how people play him, what they build, and that he's nearly useless. He needs a full rework, I don't know what this champion is supposed to be.

Gnar: I said I won't talk about champions that have only small problems, but Gnar is a special case, since his mere existance invalids a whole roster of champions, and I think that's a huge problem. I have no idea what to do with him, nothing major, just some adjustment to prevent him from abusing too hard from melee champions.

Kalista: I think we all know what her problem is: passive mobility. This champion offers little counterplay once she's fed, and her presence forces you to play things like Lissandra to be sure you'll be able to shut her down later in the game. The fact that her mobility scales with items makes it even worse. And it scales with items too much: boot tier increases her dash range, while attack speed increases the amount of dashes. When she has zero items, her passive is tolerable. When she has Tier 2 boots + 2.0 attack speed, her passive makes any melee or ranged, skill-shot based champion want to punch the keyboard. Her passive is the problem. It's a cool mechanic but too abusive and toxic. It just need to be changed.

Lee Sin: And here starts the hate against me. Yep, I think Lee Sin's kit is toxic and problematic. He's not overpowered though. His problem is that his kit has too many tools to adapt to every situation really well. Lee has almost no weaknesses, and has the tools to exploit the weaknesses of every eneny he might face. I read a funny comparison in this post some time ago, and I think it's pretty accurate: Imagine Nautilus has a pack of big, high-quality shovels, and Lee Sin has a pack with different, decent tools: one good shovel, one good screwdriver, one good spoon, one good ward... If there is a competition of digging holes, Nautilus will win, but at every other competition, Lee Sin will win with no problems, so he just has to avoid hole-digging competitions. It's also worth mentioning that Lee's early game is too abusive, while his late game is too underwhelming. Even champions like Riven have not that huge power difference between early and late. Take away some unnecessary power from Lee while strengthening the rest of his power. Shift some power from his early to his late. REMOVE WARD HOPPING, IT'S NOT FUNNY.

Pantheon: That's a fun case of a champion that is useless but frustrating to play against. His spears are the first reason people suicide around the world. I can't imagine a champion more toxic than Pantheon (ignoring Darius). He needs a more interactive kit, and he should not be able to ignore turret hits (it's frustrating to get dived by a Pantheon that tanks two or even three turret basic attacks in a row). He needs a rework that keep his core identity (if he has that), but make him a more interactive champion overall. Also, stop blocking turret hits.

Thresh: I'll be clear: he has everything. Champions like Lee Sin lack diversity if you compare them to Thresh. Just take away unnecessary power from his kit. Remove his basic attack steroid (on his E). Remove the shield grant by his W. It's done.

Veigar: I would not be mentioning this champion some months ago, but since you've stated you don't like his insta-stun, I have to do it. Veigar's current state is awkward, he's too weak and no tweaks will change that. His whole kit is balanced around having an insta-stun and his "If you enter within 675 range from me, you are dead"-playstyle. Just do a bigger, better rework if you don't want him to have an insta-stun. I wrote this post some days ago talking about a mini-rework.

Vi: She invalids immobile mid mages. Nothing more to say. Reduce her gank potential against Lux.

Zilean: His skill-shot based Q just doesn't work. Sion-level rework.

Satan: He is Satan. Kill him.