Jhin's passive
Jhin's passive is a neat way to implement the "slow and meticulous" part of Jhin. But as cool as the concept is, it limits his versatility and makes him hard to balance. I think Jhin should be able to function as a long range ability-based sniper (an AD mage if you will) or as a closer range auto-attack focused marksman. Here's how he functions in those roles currently:
Sniper Jhin's ability to deal damage from behind walls or way behind the fight (with his W and R) is a major strength. He can actually make use of traps during fights since he's so far back, because enemies will have to come toward him (and through the traps) to attempt to kill him. However, since Jhin gains so much bonus AD from attack speed and crit chance, neither of which normally affect abilities, his scalings have to be low and thus buying solely AD is bad on him. He has to buy crit chance to be viable, but from so far away he won't ever use it besides for boosting his AD. Thus his damage remains low from far away because he is balanced for his abilities and attacks to be used at the same time.
Marksman Jhin has crazy attack damage and will be building crit anyway, so his auto attacks are powerful. His Q is effective for damaging multiple targets and he compliments assassins' and mages' burst with his execute. He is a high damage threat in fights as a marksman should be. However, Jhin's reload time severely hurts his DPS. Unless Jhin and his teammates all plan the fight around Jhin's reload, he will never fight as effectively as he could. Also, since Jhin is so close, it is hard to use traps effectively (and nearly impossible to use them offensively).
Here's how I think both of these roles could be empowered and clearly defined without making either one of them too powerful, all just by changing his passive a bit.
(Additions are marked like so and removals like so.)
Whisper DEATH IN 4 ACTS: Jhin's basic attacks utilize ammunition and his attack speed cannot be improved except through growth. Jhin can attack 4 times before having to reload for 2.5 seconds (drastically reduced by attack speed), and automatically does so after 10 seconds. The reload timer is reset back to 10 seconds upon attacking or using an ability.
The final shot is a guaranteed critical strike that also deals 15 / 20 / 25% of target's missing health bonus physical damage.
EVERY MOMENT MATTERS: Jhin's critical strikes deal 25% reduced damage, but grant 10% (+ 4% per 10% bonus attack speed) movement speed for 2 seconds.
Additionally, his attack damage is increased by 2% - 40% (based on level) (+ 4% per 10% critical chance) (+ 2.5% per 10% bonus attack speed) attacks deal (damage based on level) + (damage based on attack speed) + (damage based on critical chance) bonus physical damage on hit.
Here's why these changes would help Jhin better fit his roles:
Sniper Since Jhin's attack damage is no longer increased by attack speed or critical chance, his abilities' scalings can be buffed. Thus, pure attack damage will be more effective at increasing his abilities damage, allowing for a Jhin build to include attack damage, cooldown reduction, and armor penetration. He can utilize his range just like before, and now deals plenty of damage to boot. Since sniper Jhin wouldn't want to build attack speed, he would be very slow. If an enemy manages to make it past his traps, he would be pretty much dead meat if his abilities are off cooldown. His attacks wouldn't help him much since his damage is focused on his abilities.
Marksman As a marksman, Jhin will buy a mix of critical strike chance, attack speed, attack damage, and movement speed, just like any other marksman. His reload time will be drastically reduced, allowing his DPS to remain high. The on hit damage on his attacks makes sure he matches another marksman's DPS but his crits don't get out of control, all while still keeping the "slow and meticulous" feel to Jhin. Since marksman Jhin wouldn't have a ton of attack damage, his strength would be in his auto attacks rather than his abilities thanks to his passive on hit damage. This means that marksman Jhin would need to get closer into the fight to unleash his potential.
By implementing these changes, each role has clearly defined build paths, strengths, and weaknesses. Sniper Jhin has powerful range and ability damage, but is weak when an enemy reaches him due to his lack of auto attack damage and movement speed. Marksman Jhin has high DPS and movement speed which is everything needed in a teamfight, but he lacks the raw damage of sniper Jhin when fighting from far away.
Thanks for reading, and please upvote this if you think it's a good idea so Riot takes a look at it!