I got this idea. Tell me what you think.
You have normal 4 atunements but you can turn them on and off. All of them share cd. And if you have more then one element active you get new skills (probably not tied to weapon since it would be soooo many skills) based on combined atunement. But skills in more advanced atunement are more powerful but have much longer cd.
If i count correctly this means 55 new skills.
Maybe there could be no atunement (arcane?) Option when you turn off all atunements.