If so, I suppose each subclass could be treated similar to a "school of magic" or "skill tree". For example, Fire magic excels at dealing damage, Ice excels at snaring and slowing but lacks damage, and so on. Then the player can mix spells from each school/tree to have their own unique playstyle.
Or did you mean that each one is generally similar, but have different art behind their spells? That's kind of what I get from this:
they have very distinct spell effects, nature mages throw leaves and make spikes shoot from the earth while a shadow mage throw shadowy tentacles and swirling patches of darkness. The game effect is pretty much the same, damage and snare or root. But I know players love to customize.
If so, I don't think that'll make players feel like they're customizing, since there will eventually be hundreds of players with the same subclass as them.