This is not the topic, but there are some overlapped classes/skills/themes already:Except you really can't. They don't have to be the same to share a common theme. No two classes in WoW at the moment overlap in theme.
Demon hunters, no matter how you twist it, at the end of the day overlap with warlocks, warriors, hunters, and of course rogues.
Warden, no matter how you twist it, at the end of the day overlap with monks, rogues, paladins/priests, and whatever else I'm forgetting.
They aren't the same, but they're similar enough - you won't see it in WoW.
Paladin=warrior+priest
Monk= rogue+priest/paladin+shaman
Shaman= mage+priest
Druid= mage+warrior+rogue
etc...
They aren't the same, but they're similar enough - but you have all in game already, some of them using the same type of skills or the same Fx in some abilities.
But If you can give them something unique, like armour or new look, then you have a new unique class.