Magical accidents. Potions. Demon summoning accidents.
Hitman already ventures into the absurd and this would just be one more step.
Dishonored was a bit like that
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
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Getting that artist on board would definitely be a plus. Also likely to turn into an AO game but that might be fine.
I wasn’t even considering something that wacky in the concept, but damn, that would be really fun.
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.