A note on Assassins and an idea or two
NOTE: If you want to skip some of the explanation and boring context, you can skip to "Starting Point" or "Potential Suggestions", since I realize this is a lot of text.
Intro: I've made a post about this already, but I'll try to re-iterate what makes an assassin good and balanced. Obviously, every assassin is supposed to deal a lot of damage in a short amount of time. What's also obvious, is that any assassin who spends too much time in close quarters with the enemy is going to die very quickly. And because assassins aren't exactly tanky, they are very susceptible to hard CC.
Assassins always require coming into close quarters to deal damage, because after all, that's where the counterplay comes in. But, a good assassin (good as in viable) will have some way of getting out of the middle of an enemy team after going in to deal damage. Fizz is...Fizz, but he's not as good considering the nerfs on trickster drastically increased the downtime where he's just sitting in the middle of an enemy team. Zed can prep his W for escape or he can us an ultimate shadow to get out. Leblanc has distortion, and Kassadin has ultimate.
(This is not to say Zed/Fizz are in super good states as assassins right now. I firmly believe Kassadin and Leblanc are in healthy states for assassins right now, but that's a separate matter)
Now if we look at other assassins that we recognize as not being very good picks, we'll find Akali, Diana, and Talon. These champions all have the same problem: Once they go in, there's no coming out. Their kits are also a bit unhealthy, in the time they leave for their opponents to react. Leblanc and Kassadin leave time open for their opponent to react and punish them for going in. (Kassadin's ultimate cooldown, even at max level 1 to 2 seconds is plenty, and Leblanc's distortion travel time. For those who don't know, Leblanc is only an early game counter to Twisted Fate for this reason. Later in the game you distortion in, he stun cards you, you die. Butt...he doesn't get to start doing that to you until later in the game.). The same is true for Zed and Fizz. When Zed/Fizz go in, there are small openings where they can be CC'd and killed. Fizz has downtime when Trickster goes on cooldown and zed has to wait before being able to go back to his shadow. This is why Lissandra is typically good as a mid pick against just about any assassin. She has the hard CC to stop them in their tracks when they try to move in for their burst. Lissandra is just an example, there are plenty of champions that can contribute CC to act upon these openings. Of course, the openings can't be too long or else it would be pretty straightforward to target focus an assassin whenever they tried to deal damage. Again, this is the problem with champions like Akali, Diana, and Talon. There's little to no way to react to these champions' burst damage, and there's no way for them to help themselves after they go in.
Instead of being a game of skill, like with the other assassins, it becomes a game of raw numbers. If talon has the raw damage to kill you with his instant burst, then you're done. If talon doesn't kill you with that burst on the first spell rotation, then he's pretty much done. It's pretty toxic on both sides. Kassadin and Leblanc are balanced because they both deal respectable damage, lack wave clear, leave enough downtime for their opponents to react but not so much that going in for damage is virtually suicide, they have counter matchups, etc. I think Zed is only marginally toxic because ADCs have no counterplay to being ulted by him. Mid laners are at least able to use zhonyas. Fizz's ultimate not being able to be QSS'd isn't as huge a deal since he actually has to throw out a skill shot, which DOES NOT go through the tank line standing in front of you. Fizz's problem is that there's TOO MUCH downtime after he uses trickster. Seriously, it's 6 seconds with maximum cooldown reduction.
So I think Zed and Fizz could use some small changes or tweaks, while Leblanc and Kassadin are in as good and healthy a state as assassins can be. I think just about every other assassin on the list could use kit changes to fit the general strengths and weaknesses/counter play to assassins.
Starting Point:
Diana's Q is fairly long range for being able to instantly put out damage with ultimate. If you look at Kassadin, Leblanc, and Fizz, the ranges on their gap closes are either average or below average. (I consider 550-600 about average because that's the auto range of most champions). For reference, Diana's Q range is 230 more than Leblanc's distortion. And her ultimate range is only 5 units shorter. I think for Diana's case, I think the speed on her Q could be slightly increased, but the range on her Q and ultimate could be reduced. This is just as a starting point.
I think Talon would require some more thought, because his E is used as a potential escape mechanism as well as an engage tool. Still though, his E range is 100 units more than Leblanc's distortion, and his blink into his ultimate is a near instantaneous action. Leblanc at least has to take time dashing towards a location, and doesn't deal damage until reaching that location. Plus, distortion is easier to anticipate because it's not super high range.
Akali, I'm not sure if I can really comment on.
After reading Riot's announcement on the Assassin update, I'm fairly confident that they know about the issues with some of these assassins dealing instantaneous damage, amongst other things. If they don't and they're reading this, they know now. I think they know that Kassadin and Leblanc are in healthy states, but they also know that there's room for improvement on most everyone else.
Potential Suggestions:
Obviously, Riot can't give every assassin a kassadin ultimate or a leblanc distortion or a fizz trickster. The abilities ought to be different in some way. I think that it would be interesting to do a spin off of leblanc's distortion for akali, however. What if there was an interaction where she could flash/blink/dash back (Within a set range of course) to some location in her W zone if it's cast. Idea of this interaction. Akali would have downtimes where she can't get herself out of the fray, and she wouldn't be able to change the places where she dashes back to. (Like how Leblanc can use distortion twice). Furthermore, you know that whenever she sets up her W zone, she's planning on going in for an attack. This gives you a chance to anticipate that she's about to go on you.
For Diana, here's my half-idea. Let's put her E (her pull) onto her shield. Whenever all three balls of the shield triggers, she pulls all enemy minions/champions/neutral monsters towards her, and maybe we give it a slow or maybe we take the slow part away. Then, give her SOME ability to replace her previous pull that will make her more unique than pressing buttons and going in. This way, she can't just go on you from super far away, pull you towards her, and slow you as well. In the middle of a team this may prove to be more useful, since there will be more champions and the shields will go off all at once. But from a solo perspective, you will have some time to react while t he balls rotate around her. (It takes time for them to all trigger on one target).
For Talon, I'm honestly not too entirely sure of where I might start to make changes. I think it might be questionable to nerf the range on Cutthroat, so I would probably consider changing the potential for FULL instant damage on his ultimate. For those who don't know, Talon's ultimate does damage with each blade that hits an enemy, and it does damage the same way with rake -- when the blades get thrown out and when they return to Talon. My idea is to keep the stealth part of the ultimate, but to change how the blades come back to him. The ultimate can still be instantly cast, but the blades won't return to him until after a few seconds. If Talon uses an ability after using an ultimate, his stealth will still break, but the ultimate blades still won't return until the set time has expired. If talon dies, then they will eventually return to the spot he died in. This way, Talon will still be able to deal good damage, but there will be more skill in how he maneuvers himself to get the maximum damage off of his ultimate, and those playing against talon will have a chance to react to the second half of the ultimate's damage.
I don't really have any elaborate ideas for the other assassins, but this is what I just happened to think of for 3 of the assassins that I think really need some changing.
Thanks for reading this wall of text!