Simple mechanic riot should impliment...
Cowardice.
So tired of champions who can all in you at level 2, then run off like nothing happened, without getting a kill, or taking any damage, or pulling aggro (Malphite, fiora, ekko, panth... to name a few). They just harass, and harass, and harass, and never get punished for not just finishing the job, in other words, being a COWARD.
The mechanic would be very simple, if you go in on an enemy champion, any damage at all, minions take note, but don't attack, yet. If you continue to jsut poke at, but not fight an enemy champion minions will immedietly stop attacking your minions and attack you with increased damage and speed, punishing you for BEING A COWARD. However there is more, if you deal damage to an enemy champion, and they fail to hit back within a second or two, or you turn your pack on them, the heal for a portion of the damage you dealt instantly. Punishing you for refusing to fight, this heal depends on how much damage is dealt, and how leveled you are, example if You are playing AD panth and gut your opponent for 50% of their HP in a combo whiel 2 levels up, then run off like a pussy becuase you refuse to commit, they regain 80% of the damage you just did becuase you refused to jsut finish the job, or even risk fighting, instead deciding to play like a coward, then claim you have skill, becuase you know in a fair fight you would lose.
This can apply to many champions, basically a punishment on people who insist on jsut being poke based champions who get to harass endlessly without ever pulling aggro, this has become such a rising problem, i finally snapped when a malphite ulted under my tower and 2 shotted me after dominating lane, without ever once autoing or commiting to kill me, and the tower did not bother to aggro him because i died between shots, so fast it couldnt even register me as taking damage. And again after facing a pantheon in lane who literally combo'd all in every 5 seconds, then ran like a pussy instead of jsut killing me, becuasehe knew in an extended fight he would lose.
So... yeah... how abotu SOEMTHIGN to punish poke champions who never take minion aggro, or players who play fighter/juggernaut/dualist champions without actually fighting you? Ergo, being cowards?