The Problem with Rengar is a Problem with the Identity of Assassins

Riot has said that assassins are supposed to get in and out of fights after killing targets that are out of position.

The problem with old Rengar is that he would 100-0 people out of stealth.

Assassins are supposed to kill carries by 100-0ing them. If they kill the carries to slow, the carries will get healed, shielded, peeled, or lifesteal. Kill them too quickly, and the carry feels that it is unfair. Kill them too slowly, and the assassin feels cheated. This is the problem with the 100-0 part. The assassins offer no utility or tanking for their team, only dmg. Their goal is to 100-0 carries, as quickly as possible. Without this, carries would run rampant.

As seen here by Riot, they wanted to make assassins more fair by increasing their skill ceiling. The problem with this is that, when you get someone who's good at that assassin, the target will get even less time to outplay them, because the skilled player will be able to fully utilize their kit to kill the target in an even faster time than an unskilled player. Once again, look at Old Rengar. He was relatively easy to pick up, but hard to master. If you didn't do the mid-air Es, or triple Qs, you could still kill people, but it would take more time. Once you mastered those mechanics, you could kill the squishy targets in less than half a second. If Riot were to want to balance assassins based on skill ceilings, they would have to balance around the high-skilled players, treating the playerbase in a similar way to Yasuo or Riven. That tends to not only make the champion unattractive to newer players, but also tends to give the advanced playerbase a bad name, while still having that "unfairness." Purely raising skill ceiling does not fix the game health problem of playing with assassins, but only makes it more interesting to learn how to play as an assassin.

So, Riot forcefully extend the burst time of assassins, adding longer cast times, cast animations, delayed abilities (LB passive, Talon W), etc. The problem with THIS approach is that it messes up the skilled playerbase, who feel cheated for mastering their champion. Before you can even get out your burst combo, you get ccd, or your target gets shielded or healed. This provides the people playing against the assassin more counterplay, but the people playing as the assassin a feeling of being cheated. Riot, once again referencing, the "Road to Preseason: Assassin Update" post, wanted to give the ability for assassin targets to "fire back." When assassins go in for a target, however, it's always one-side. Either the target doesn't fire back and dies, or they fire back and the squishy assassin gets blown up (when the assassin can't get blown up even when the target fires back, you end up with bruiser or tank assassins: Tank Fizz, Tank Ekko, Bruiser Rengar). There can't be a grey area for firing back. There should not be reactive measure to an assassin going in, as there should be preventative ones. This is why extending the burst time of assassins usually will backfire gameplay health wise.

By doing a combination of increasing skill ceiling and burst time, both sides would be appealed. But the right balance needs to be struck; too long of a burst time makes the assassin not feel like an assassin at all, and more like a bruiser or a mage. As a result, people will build for those champs in their new, redefined, better class. This is the case of LB and new Rengar. I feel like what has been done with assassins, has been less of increasing their skill ceiling, and just adding more, new, complicated, "stuff." The new assassins don't really feel more mechanically difficult than before; they just have new toys to play with, becoming "flashy," as Riot would describe it.

The skill ceiling of assassins needs to be further raised, and the burst times need to be adjusted individually for assassins.

The 2nd part is the "out of stealth" part. Assassins need ways to get to their target, and to get out. But they should not be the ones engaging fights. That is what many assassins do, especially when ahead, and what Rengar, both in his current and past iteration, does. Assassins wait for the carry to be vulnerable, and then kill them. More emphasis should be put on this.

I think assassins were most healthy when you had preventative countermeasures to their engagement, back when you could have much more vision on the map.

While you could try to balance assassins kits individually, it would be very difficult to appeal to both the carries and assassin players. The quickest, best solution, would be adding in preventative measures that can be used by everyone (encouraging team effort to defeat assassins) by having more vision once again on the map. This could also possibly help with the issue of constantly ganking or camping junglers; increase the ward limit (but don't make them unlimited), and try to make it easier for everyone to get wards on the map. Supports should not be the only ones warding.

(Wards shouldn't be unlimited, because that encourages ward-botting, as one person just wards for everyone. A ward limit needs to be there, but it should be higher, so there can be more wards on the map overall. There needs to be an easier way to get wards; is a good idea, but is relatively unused. Bring tracker's knife-types of items to other laners so the map can be filled with wards, and the assassins need to be more careful about their positioning, when to go in, and when they can come in from an unwarded area)