I've always liked the idea of possession, so I thought that if I make a warlock whose boss is parasitic.
The customer can only own the host if it is compliant and it is part of his pact. As you level up, the client controls the host harder and harder, eventually taking his whole body.
To be clear, I would not give it new capabilities or new mechanisms outside of the role-play perspective of having another entity take control of your body. If I can not do that, can I do something with the principle of slow possession?