For those of you who are unwilling to abandon Nazis in Science Fiction, it seems that an English girl named Unity Valkyrie Mitford boinked Hitler and, after a suicide attempt, returned to England where she gave birth to a baby girl in secret.
She was a complex and interesting character, obsessed by Hitler, who stalked him and eventually seems to have been his lover.
I think there could be an interesting story about Hitler’s daughter, born in 1940, who runs a secret fascist organization. As much as I hate Nazi stories, there could be an interesting plot here.