onCrit trigger and/or condition
Speaks for itself. an onCrit trigger to fire skills upon critting. We can add general skills to it through the stats.yml but there is no way for us to make certain weapons or armorpieces add additional critical effects.
Additionally, a condition or skill var within the onAttack and even onDamaged triggers could allow skills to conditionally respond based on if an attack was a crit or not, opening quite a few options for designs.