I think that attacking a more than 3 lvl higher char when it is vunerable (like when attacked by someone else at the same time, low HP during grinding, AFK etc.) Is not quite risky at the time being, because when the attack is cancelled or the attacked dies, he/she won't be able to fight back/take revenge.
I suggest therefore the rule of revenge: the attacker would become kind of an outlaw, but selectively: only attackable by the previously attacked/killed higher lvl character (maybe its clan members too?). The duration might be shorter than normal outlaw status, maybe 1-2 days is sufficient.
It happens sometimes that someone attacks another char unintentionally in crowded areas - damn touchscreen

So maybe this rule should apply only if significant damage was caused, like 20% of the total HP or kill or something.