Dishonored, but with larger levels, more tools, not just magic powers, thus more opportunities for accidental kills. Set pathways and activities that the targets use. Some way for possession to transform into your enemies to engage with social stealth and hide in plain sight, or invisibility. I think OP's idea of potions has a lot of potential there.
A nice mix of different types of wards against certain magic in certain areas and magic detection spells with different traits could definitely make it interesting.