Dishonored was a bit like that
Gaming
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.
Thief 4: The Dank Age
So basically, Hitman in the world of Oglaf?
That would be pretty fun, actually
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.
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.