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.

I wasn't even considering something that wacky in the concept, but damn, that would be really fun.

Getting that artist on board would definitely be a plus. Also likely to turn into an AO game but that might be fine.