It seems as though the villain will come from the World War II era, at least based on the information presented. Here are my educated guesses:
What better person to use than the master of hate and fear himself. Yeah, he has been used quite often before, but only in clone form as the Hate Monger. Would be interesting to see.
The doctor's nickname was the "Angel of Death." Big time creepy character, would be interesting to see him involved.
3...Unknown Nazi mystic.
It is no secret that the Nazis were interested in and dabbled in the occult. Not a stretch to see someone who survived the Second World War sowing the seeds of fear and hate throughout the world unseen since 1945.
4...Hydra or a male Gorgon (from Greek history).
Not the organization, but with a backstory where the organization was formed to worship an actual being known as Hydra. If you go to comicbookresources.com
and check the link, you'll notice snakes in his hair.
I loved the series from the 1970s WAR IS HELL. It was one of the first books that Chris Claremont worked on and the storyline with Death was simply amazing. Now that Kowalski is an embodiment of Death, it would be interesting to see a return. By the way, I think Marvel dropped the ball by not including him on the God Squad in the Chaos War. The God Squad was made up with demi-gods, or half-human gods. Hell was represented with Daimon Hellstrom, but not the abstract of Death itself. Would have made a powerful ally...just a side thought.