Short of simply nerfing them till they don't assassinate properly 
, is there any way to truly design a "healthy" assassin? Given how almost every assassin in League has received complaints at some point in history 
... can burst+mobility ever be implemented in such a way that it feels "fair" to both sides or is the entire concept of an assassin doomed to never be healthy by design?
In my opinion, a health assassin is defined as one that doesn't kill you because you stepped too close to them. I'd also contend that the healthiest assassin kits don't involve mashing QWER (+items) as fast as possible in a given order.
A somewhat unpopular champion, but one I think is actually a decent Assassin is 
. Lets face it, if Shaco was a "Mash QWER to win trade" champion, he'd be a far more common pick. He has to chose at every moment whether to hold onto an ability or use it now. Even if he tried to use every ability at once, the delay on his W and R makes it so he can't. This forces Shaco to find an advantage that he can actually capitalize on, rather than just trying all of his spells with a few auto attacks thrown in and seeing if that happens to kill the target.
Shaco's certainly not weak. He can do his job (much to the frustration of the enemy team), but he doesn't become a 100-0 no counterplay champion any faster than any other fed champion in the game.
Theres only really 1 thing an assassin has to be in order to be "healthy"
your main kek
A healthy assasins shouldn't have all the burst failproof and totally instant(like talon and to some extend akali), and if it is a 100-0 pattern, it shouldn't be to reliable and needs to give a certain window for the opponents. The classical idea of an assasin coming out of nowhere, instagibbing a target and vanishing again simply can't be apllied to lol, as a fast 100-0 burst never is healthy. See talon or rengar that prettymuch follow that sterotype... As a result, assasins need fallback patterns, things that keep them relevant if they don't oneshot a target, that allow them to go for a second round to finish the job. They need tools to stay alive/get out of a fight and in again without to much downtime without relying on raw tankiness. Core for that imo would be looking at stealth as a **defensive **mechanics, mostly removeing the hardcounter of truesight. Offenisve stealths like rengar or twitch would need to be changed to something that has more counterplay, akin to eve's passive.
Kah has great potential to a healthy assasin, especially with the lower cds on pbe. What he would need is lower damage vs isolated to prevent the oneshots, but therefor some buffs to the ult, increaseing the duration by 0.5sec, reduceing the cd between ult casts or adding a stack system, and maybe a buff to his base tankiness or W heal. But with his passivet and the low cd Q, he has the tools to kill targets with and extendet pattern rather than a oneshot combo, and with his ult he can avoid focus between the Q cds.
Zed also has great potential to a healthy assasin, if his E would deal a bit less damage but have 3sec cd, meaning he has less burst and waveclear, but can even without alot cdr land a second E duering ult, and if his Q would have less damage but deal full damage for aditional hits from shadows, so he actually needs to hit double shurikens for alot damage, which adds more counterplay to his burst and limits his poke/waveclear. Maybe even bring back the 1 sec delay on the ult re-use. As a result, he should have to either land multiple shurikens after ult or stay around for a second E for the kill combo, which allows more counterplay, and his laneing should be less powerfull due reduced poke and waveclear.
I also think the damage of ranged champs needs to be toned down overall, if you go in as an assasin and don't instagib your target right now, you most likely die 2 sec later if it even needs that long.
As for right now, i'd say ekko is the healthies assasin we have, unless really fed, he does not burst 100-0, he relys on hit n run to get opponents low, baiting him into a ult+w for full impact, though the stunduration and ult damage certainly could be lowerd in favor of some other buffs. And even when he's fed enough for oneshot, he still needs e+q+secondQ or another AA, which gives a small window to dash away or cc him. So all in all he fits the bill, no 100-0 pattern but enough tools to somewaht savely stay around for a second round. Only the mass destruction potential of his W+ult is imo a bit to much for an assasins.
First, it's never going to be enjoyable to play against an assassin, and if the assassin can do its job, it shouldn't. The class is entirely built around killing the other person.
But balance? Yes. It simply needs to have enough counterplay in order to fight against them. Zed is a pretty good example of this. His assassination pattern is one of the easier to counter out of the entire class.
Assassins by definition don't have much counterplay if they make a skilled play and do their job.
All the complaints are because no one likes getting assassinated, and no one ever will.
That being said, zed does have to much safety and spammables and talon is insanely hard to deal with if he gets ahead. Not huge issues just anoyyances on top of already potentially anoyying champs..
's ult cant even kill the target
The consumption of lots of oranges will make them healthy
Any champion that can 100-0 you coming from fog of war before you could possibly react fundamentally lacks counterplay.
There are a lot of healthy Assassins bust most of the player base is garbage at this game and has no idea how to deal with an Assassin so they are all unhealthy
The real question is what makes for a healthy tank?
I'm surprised 
hasn't been brought up since she's so fun to play against LOL.
But I think Katarina is about as healthy of an assassin that we have
She has clear strengths, clear weaknesses, times where she can shine and times where she falls into darkness
Imo, a healthy assassin should have a degree of reliability in their kit that lets them contribute as an assassin even when behind, but also a degree of outplay potential even when ahead. Ideally, when an assassin's trying to kill you, they should be presenting you with a series of quick, high-stakes challenges where your success or failure will make a clear impact on the outcome.
While Ekko might have some kit overload issues, he does a lot of that already: his Q encourages you to outmaneuver him, his W can be outplayed through prediction and quick reaction, and his ultimate can be avoided by keeping track of his image. His abilities are low-cooldown enough that he can always try going for the kill even after burning through his rotation, but manage to still be meaningful due to how interactive they are, and also let him be useful even when behind (though not necessarily as an assassin).
By contrast, Rengar is probably the worst example of an assassin right now, since his own kind of burst (i.e. "throw everything at your opponent from invisibility within a fraction of a second and hope they die") has little to no outplay potential at all, and ultimately ends up making his success ultra-binary.
Kha may have a few issues, but he's pretty close to being a healthy assassin, as he alternates between moments of high power and windows of vulnerability during fights.
A healthy assassin should not be able to kill you in the middle of the team and then just get away with it.
They should pick their targets precisely and be more focused towards stealth and ambushes rather then just, BOOM I entered your vision, now it is grey.
Also thye should have counterplay in that they can't contribute much to a teamfight, are squishy and can't get away if they are not able to kill their target quickly.
assassins like and are able to escape when they fail to kill their target.
Can't have a "healthy" assassin. It's feast or famine. To snowball or to feed.
Yi isn't an assassin, he never ward.
Akali isn't healthy due to 3 point-click gap closers and due to it is heavily nerfed atm.
Zed is healthy has good counterplay while still being able to assassinate a target if allowed to.
Talon is healthy as well, has a much cleaner assassination than zed and even lacks ways to get out easily if he fails.
Ekko should have been a bruiser(renamed to fighter) tbh, as an assassin he has way too much CC even if it's an extremely conditional one.
Rengar isn't healthy but not due to him being an assassin but how feast or famine he is. If they removed bonetooth necklace and moved some power of the item into his kit itself(Maybe like kha's evolution points) He would be a MUCH healthier champion. (Personally imo they could also give kha+rengar a passive evolution point so for kha his "Taste their fear" would deal damage to nearby winions/monster near the main target. If this was done for both rengar and kha they'd still have their lore vs each other and rengar would be more consistent and does easier to balance.
I would say someone that can get in and out easily like or and also does high amounts of damage.
do you consider ekko an assassin?. I mean hes more of a fighter honestly.
Things an assassin needs to be healthy;
A burst of sustained damage. Not burst damage and definitely not sustained damage - it needs to be a small window of high dps. If an assassin is able to unleash their entire damage potential in a short period of time, it is not health. However, if it takes an assassin 2-3 seconds before they can kill someone - then there's time to react allowing sufficient counterplay. Bonus points of damage can be mitigated through various means (such as dodging a skillshot or playing around Kha's isolation). Energy is a good mechanic for it, as well as Zed's W and R.
Telegraphed tools to do their job. Remember when Rengo used to be able to just jump out of nowhere with his ult with absolutely no warning? Yeah, that's an issue. You need to be able to read an assassin and have an inkling when they're about to all in you. Zed's ult is a wonderful example of this.
A viable fallback pattern with drawbacks. Remember when games where about assassin splitpushers, that would kill any single person who went in there to stop them, or even probably killed 1 of 2 people while escaping? Bad. What is good would be a sort of poke or team utility (but it can't be too much - Yasuo and Ekko show what happens with too much utility). Zed is able to expend a large amount of resources (W CD and energy) to poke (which can be avoided and leaves his damage potential crippled). Having a reliable method of escape is acceptable as well, since they are still able to contribute to a fight - as diving in still provides ample risk and reward.
I think one of the few health assassins is
Shaco's kit is very bad if you don't play him correctly. His stealth mechanics and ultimate make his effectiveness 100% mind games. Yeah, he has some kind of unfair tools, on some people's opinion (his boxes come to mind) but really, he uses predictability and what a normal person would do in order to deceive you and, ultimately, kill you. His passive Backstab damage relies on your ability to sneak up to them, and the predictability of them running away from you so that you can continue to do more damage. His boxes need to be placed correctly, and his ultimate needs to trick people into attacking the wrong Shaco. This is a healthy assassin, because you should know what he does, and there is a clear way that you should not play against a Shaco.
Champions like Talon, on the other hand are really hard to gauge how to play against them except hugging the tower and farming until someone else comes into lane. Say what you want about his pre-6, pre-BF sword damage; once he gets it, it's very likely that he can combo burst you down with little there is you can do about it. You can blow your escapes, you can blow your summoners, but without Zhonia's, he's going to chunk you, and probably kill you. Even then, if he out farms you, he's just going to dive you anyway, so you better hope you build zhonia's before he finishes duskblade, bf sword, or whatever his first item is. Your tower shouldn't be all powerful like the fountain laser, but it should at least be able to punish someone who's trying to kill you when you're at full hp standing directly under the fucking thing.
Another thing that should not happen, is an assassin that can stop building damage, switch to tank items, and still do that signature assassin damage. See bruiser Yi. Otherwise, he would be pretty healthy. Simple, kinda easy, but not that bad if he couldn't wreck everyone with just rageblade and sated devourer, then become unkillable because he's also built some random shit like randuins, spirit visage, and titanic hydra (I wish I was kididng, but I'm sure I saw something similar to this).
A healthy Assassin is one you can see coming, if you try.
With Assassins, you need to be one step ahead of them. If they catch you (Assuming you're a squishy), you're dead. The game is keeping them from catching you, using a combination of vision, clever map movement, and teamwork. That's the teamwork, and the reward is either A), when you catch the Assassin trying to hunt you and kill them first with your team, or B), when your efforts make them helpless in teamfights because they cannot kill anyone.
A "healthy" assassin would be a champion that can go in, assassinate a carry, get out alive, and then have to wait on significant CDs to be able to do it again.
If you can't win a 4v5 if the assassin does that, you don't deserve to win the game.
It's only doomed in "league of ADCs" Where you have ADCs in every lane including jungle and a few supports to peel. (See Janna counters all melee)
if an assasin cant comfortably one v one champions why should they be in the game?
A healthy (but GROSSLY OVERPOWERED SO IT WAS RIDICULOUS) assassin in my mind was end s3 early s4 , a lot of his damage was locked behind clean-up, missing health and isolation (granted iso range was pretty small back then) making him having to pick and choose his moments.
Now, because as i mentioned, he was grossly overpowered, he could brute force his moments against many champions, but the concept of waiting for them to split up as well as not being able to jump in anytime anywhere unless it was the adc split pushing (don't do that, seriously don't do it, i see it too much) while waiting for a clean up to do most of his damage sounds reasonable to me.
Well, they saw fit to nerf Anivia and buff Leblanc. If at any point this happened in the past, people would have rioted. Anivia did not need a nerf, Leblanc didnt need a buff. An equal skilled Leblanc vs Anivia will almost always result in the Leblanc surviving the initial burst with her clone passive, anivia will egg form and assuming leblanc and niv are equal in strength, lb will kill the egg before the rebirth trigger.
Point is, not enough counter play.
