One thing about a WW2 setting that worries me is a potential alliance between the allies and the nazis against the Predator. People think a Human-Predator alliance is bad, what about the former? I would so hate for nazis to be portrayed as morally superior to the Predators just because they're human like the allies.
I think they should instead make it a triple threat match. No alliances, just the Predator, the allies and the nazis killing one another.
Anyway, I agree with Crazy Shrimp about the Predator killing Hitler.