I agree with spider. Anything that's not focused on the main plot and characters of the book would be good for me. I personally would like something post-war, perhaps a new band of Death Eaters decide to threaten the muggle community. Or, going for a sci-fi/apocalyptic feel here, muggles have figured out a way to be immune to magic and decide to capture wizards, using them for twisted experiments. We'd be their prisoners and try to figure a way out.