Like a "20 Questions" game that finds you the perfect next book! It's fine to say I want something adventurous, but what if that led to a "do you want your adventure to involve a body of water?" And then maybe "how many main characters are you interested in following?" I understand this would take a lot of research, but readers can add information like that to their reviews. Or maybe they can opt to take a quiz when they've finished reading, providing loads of information to provide to the database. I think that feature would add a huge perk for subscribers!