Summary: For any reason of your choice, Snape and Harry find they have to (or are ordered to) rescue Grindelwald from the Nurmengard. (Hint: Maybe he's the only one who knows of a power that can destroy Voldemort) Now, they're stuck in the wilderness with a good-natured old man who has (possibly selective) memory loss, and no way of contacting civilization.
Use any plot that comes into your head! This is an idea I have never seen before in the fandom, but would love to. Completely your choice on whatever characters/settings you use apart from the main three!