I played a game that had a character that could convert other characters into...drones. Those drones had the same moves except for one that is their converted unique skill.
Also, it's possible to place features on actors to make them unique even with the same classes, like choosing a race and then a class in an MMO. For example, two knights, but one of them can equip a gun as well. But yeah, you might need a sub-class for characters to learn skills specific to them.
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1. As said above, the simplest way, assuming only class skills are learned, is simply to give the actor racial or unique abilities. For example, everyone can be priests, but the Chosen has Bless.
2. For each actor, there would be a list of classes, with each group of class also adding skills for that character. So if you have 3 actors and 3 classes, there would be 9 classes total.
3. Find a Subclass formula.
4. You could try out Himework's scripts:
A.
Feature Conditions - Ideally, you have one class and the feature is a skill. Then you would have something like a.id == 1 and a.level >= 5.
B.
Learn Conditions - This one is more tuned for skills. Again, probably write it so that some skills can be learned if that is the right actor.