What villain would you write into one of the games and why?

Heres my pick, Dr. Phosphorus.

Honestly he could work as one of the assasins in AO, and you could change Electrotioner with him, or... Just add him as another assasin. His Boss fight would probably be around dodging projectiles and freezing him in order to deliver safe punches and not die of radiation. There could also be a radiation meter for the fight and we would need to keep our distance in order to lower it. What's your ideas?