The way you'd differenciate classes similar to old school jobs is the traits they get access too..

For Instance.

A Monk would get the "Kick Attacks" Trait which would add a Kick attack to random abilities that are Familiar to Monks.

A Warrior would get "Double Attack" trait which grants the chance to add an extra attack onto the end of any skill that Warriors are Familiar with.

Ninja would get a Trait called "Ninjitsu" Which would turn skills like "Fire" into "Katon" when equipped.