Riot can learn something from Stunlock Stuidos about "Assassins"
If you have never heard of a game called Battlerite (Created by Stunlock Studios) then I think you should definitely check it out as it is currently Free 2 Play on steam and combines the best elements of League and overwatch and puts them together.
Anyways lets move on to what the topic of the discussion is. Croak.
Croak is the name of a character in Battlerite (which has a slightly more cartoon-ish quality to it's character design) and he is a frog humanoid assassin (like an argonian from Skyrim). Now "assassin" is not an official class in Battlerite, as matter of fact Croak would be the only champion in the game that feels like an assassin, which would make him seem that he would be broken as hell.
Battlerite take champions that are similar in style to League champs and puts them in arenas to fight to the death. There are no minions, no towers, no xp, no leveling up, no gold, no buffs, no item builds etc. Each champion enters the arena with all of their abilities and fight to make it out. There is no poking an assassin down in lane to make them passive, no ganking to get early kills, no barrier or exhaust to stop their all in, no stopwatch to negate their all in, and no gold farming or xp farming in order to get ahead of an assassin. All of this points to an assassin being broken as hell but Croak actually feels pretty balanced. Here is why.
While not labeled an assassin Croak FEELS like an assassin, in part, due to his visual design. He is a frog humanoid, wielding two swords that look like long daggers with a sorta mask/helmet thing on. One of his alternate outfits looks like a cross between a daedric set of armor the nightengale set from skyrim.
The next thing about Croak that satisfies that assassin itch is his actual kit. In Battlerite you have 7 abilities instead of four and a passive so there is a bit more fluff to be had but I'll try to break it down as simple as I can. He can apply toxin to his blades, making them glow a poisonous green and apply a DOT effect to his targets, he has a medium ranged leap which can be recasted, a dash where he spins his blades around (think tryndamere's spinning slash), can spit poison at range to slow targets, gets a boosted attack speed at the start of fights, and can cloak himself, causing his next basic attack to stun. All of this attributes to his ability to trade with various targets in short fights and dance around them with his mobility spells.
The unique thing about calling Croak an assassin in League terms is that he doesn't have the ability to jump into a fight, completely annihilate someone, and then jump back out. Instead what he has to do is utilize his various mobility spells to close the distance to his target, use his attack speed buff to trade, and then leap out, OR use his cloak to run in, stun them, chop at them a few times, and then jump out before they (or their teammates) can retaliate. From a League standpoint, he technically doesn't qualify as an assassin because he doesn't possess the ability to instantly murder someone, but what he does possess is the ability to dance around his target, make small trades, or apply crucial CC so that your teammates can focus on the same target and this satisfies the same itch that playing an assassin champion does (or should). You are not just jumping at their back-line, pressing three or four buttons in a particular order, and then getting the hell out of there before you die. You are carefully kiting and whittling down your target with precise planning and forethought.
On one hand, it is balanced for people to play against because if you make a mistake against him you aren't instantly murdered. On the other if you play as him and make a single mistake you aren't incinerated because your mobility spells makes it hard to pin you down. And that is the problem with assassins in League, is that no matter what buffs or nerfs are applied they are either going to be too good and will eliminate adc's and mages without a thought or will be so bad that no one wants to play as them. Stunlock looked at the assassin idea and focused on the mobility concept and instead of saying "an assassin can kill someone in an instant" instead stated "an assassin can do a fair amount of damage in an instant" and it's working pretty well for them so far.