Now I understand the last thing you guys want to hear is that an ADC needs a buff. That being said a lot of adcs need nerfs however I feel Jhin is underperforming for a particular reason.
He's played as an assassin that can execute low health targets essentially. He's not very good with dps due to his slow attack rate.
So I would propose that the crit reduction penalty be reduced to 20% rather than what it's currently at, at 25%.
Other than increasing his attack speed, which I feel would ruin the feel of the champ, this seems like the way to have him have relevant dps as opposed to an execute machine, which doesn't feel like you contribute much to the team.
Alongside this we can possibly revert the Q AD ratio buff if needed to keep him in line.
Of course this is assuming people actually prefer an adc that's just there for executes but again it doesn't really mesh well with what an ADC does IMO.