If there are going to be add ons and new classes here, then I would advise expanding the combat mechanics.
So far, all of them have their resources before combat and mine them in battles.
I would like 1-2 other types of resources in combat.
Namely, if a basic resource is generated first during combat, then it can be used via skills. So the skills need a resource first.
This can be generated via dealing damage, or taking damage e.g.
This leaves some room to have different approaches here.
So you can give this class basic abilities, like a spell palette that charges the casting character with resources and that are used up with powerful spells.
You can do this with melee characters as well, and in addition you could also give classes that act as tanks charges only when they take damage to then activate more powerful abilities, like shield blows, counter blows, or defensive protection abilities.
You can also combine a mix of melee and spellcasting in this way for a new class.
You charge up something with melee that you can only take down with magic, or the other way around.
With such concepts, the question of the necessary rest then arises.
Thus, an additional resource is needed, which as a basis ensures that one must rest for it, in order to then be able to use the other abilities again on the basis of this resource.
For a melee fighter this can be a stamina number. After X missions, or Y fights, the fighter needs a break, otherwise most of the abilities can no longer be used.
For a mage, this would be e.g. Focus or Mental Power.
This would bring a little variety to the combat mechanics and distance the classes a little further from each other, which leads to more depth in the classes.
You can also extend something like that to e.g. servant classes.
A shooter with a pet companion, could be designed here so that the shooter allows the pet companion special attacks through ranged or melee hits, and vice versa, the pet companion weakens the cover of the target, allowing the shooter to perform special maneuvers on his part.
So just make the classes work a little more differently in combat.
What do you think about this?