How about an app that allows you to compile a list of things you want to accomplish before you die. You can share your items and browse those of others. Each item can be commented on by anyone. There could be a list of popular items, powered by the HN algorithm iteself (http://amix.dk/blog/post/19574).
I have a pretty good idea on how it would be designed, both in interface, and technologically.
Tech: RoR, Coffeescript Neo4j JRuby (required for neo4j) Heroku (see http://blog.heroku.com/archives/2011/08/25/java/) oauth2, FB Canvas, twitter connect Twitter Bootstrap
Also, jquery mobile is a piece of cake if we want a mobile site..